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{{Era|H1|H2|H3|TF|FS|GOO|HGN}}  
{{Status|Canon}}
{{seealso|Halo: The Flood}}
{{Disambig header|the life form|other uses|Flood (disambiguation)}}
[[Image:Flood2.jpg|thumb|300px|The Flood attacking.]]
{{Species infobox
'''The Flood''' (sometimes called '''The Parasite''' by the Covenant) is a species of virulent, parasitic organisms that thrive by consuming other carbon-based, sentient lifeforms of sufficient biomass. The Flood appear to have a hive or communal mind, possibly under the command of one leader.  
|name=Flood
|image=[[File:HFR The flood.png|350px]]
|latin=''Inferi redivivus''
|classification=
|subspecies=
|diet=Parasitic
|height=Variable (depending on host)
|weight=Variable (depending on host)
|distinctions=Variable
|lifespan=
|homeworld=Extragalactic
|techlevel=Variable
|notable-person=*[[Gravemind]] / [[Primordial]]
*Notable infectees
**[[Captain Keyes]]
**[[Prophet of Regret]]
**[[Wallace Jenkins]]
**[[Minister of Etiology]]
**[[Bero 'Kusovai]]
|othernames=*"''LF.Xx.3273''" ([[Forerunner]])
*"Parasite" (Covenant)
*"Shaping Sickness" ([[Ancient humanity|ancient human]]){{Ref/Novel|Pri|Page=53}}
|languages=
}}
{{Quote|One single Flood spore can destroy a species.|[[Rtas 'Vadum]]{{Ref/Reuse|Floodgate}}}}


Next to nothing is known about their origins, but it has been revealed that the [[Forerunner]]s discovered and then fought them until the Forerunners were overwhelmed. A thousand plans were tried and failed, faced with the threat of the Flood spreading throughout the Galaxy and consuming every sentient being, the Forerunners constructed and activated the [[Halo]] system, killing all sentient life in the Galaxy. The [[Covenant]] view of the Halos is one of great reverence and all Forerunner artefacts bear deep religious significance. They see the activation of the Halos as the final step of their "[[Great Journey]]".
The '''Flood''', designated as '''[[LF.Xx.3273 Flood Super Cell|LF.Xx.3273]]''' by the [[Forerunner]]s ([[Latin]] '''''Inferi redivivus'''''<ref name="bestiarum">''[[Bestiarum]]''</ref> meaning "the dead reincarnated"<ref>[http://carnage.bungie.org/haloforum/halo.forum.pl?read=865888 ''Halo.Bungie.Org'': Updated and abbreviated list, errors amended]</ref>) and referred to as '''the Parasite'''{{Ref/Level|H2|[[The Heretic]]|Quote=Thel 'Vadamee: ''"Noble Hierarchs, surely you understand that once the parasite attacked-"''}} and '''the infection'''{{Ref/Level|HW|[[Relic Approach]]|Quote=[[Re'gish Wamik]]: ''"What if the infection defiles the [[Relic (location)|relic]]?"''}} by the [[Covenant]], is a species of highly virulent parasitic organisms that reproduce and grow by consuming [[sentience|sentient]] lifeforms of sufficient biomass and cognitive capability. The Flood was responsible for consuming most of the sentient lifeforms in the galaxy - including the vast majority of [[Forerunner]]s - during the [[Forerunner-Flood war]] in ancient past, prompting the [[Great Purification|activation]] of the galaxy-sterilizing [[Halo Array]] in [[97,445 BCE]].{{Ref/Level|CE|[[Two Betrayals]]}}


The Flood were kept by the Forerunners for scientific study on many of their now abandoned structures including the [[Halo]]s. Although their intention was to wipe out the Flood by destroying everything of significant bio-mass to support them and consequently starving them, the fact that the Flood still exists indicates that the Forerunner's plan may have failed. In [[The Library (Level)|The Library]], [[343 Guilty Spark]] does mention that some specimins of The Flood were kept after the activation of the [[Halos]] for study.
Originating as a corrupted form of the galaxy's most ancient custodians (the [[Precursors]]){{Ref/Novel|Id=precursors|Sil|Page=173-175}} the Flood reproduces and spreads by infecting other organisms, hijacking their bodies and nervous systems in order to transform them into one of many specialized forms. Through infecting sentient lifeforms, the Flood assimilates their memories and intelligence, and the species becomes collectively more intelligent as a result. Because of their frighteningly rapid growth rate, as long as enough hosts of sufficient biomass and intelligence are available, the Flood is effectively unstoppable.  


== Forms ==
All Flood forms share a single consciousness, which becomes progressively more intelligent and sophisticated with every sentient host that is assimilated. When enough hosts have been infected, the Flood consciousness becomes a [[Gravemind]] - a self-aware, highly intelligent entity that strategically commands all Flood forms in an outbreak. Due to this group consciousness, the Flood as a whole is more accurately described as a single, [[Technological Achievement Tiers#Tier 0: Transsentient|transsentient]] superorganism rather than a collection of individuals directed by one or more controlling beings. In the absence of a Gravemind, the Flood possesses only basic coordination; they are incapable of forming complex strategies, and focus solely on attacking and infecting nearby sentient organisms. This stage of a Flood outbreak is known as the [[Feral Stage]]. At this stage, the Flood seek to create an assembly of biomass, calcium, and nervous system reserves, which leads to the creation of a [[Flood hive]] and the beginnings of a [[proto-Gravemind]]. The nature of the Flood's collective consciousness is similar to that of a hive mind, the Flood act as a unified entity, with no individuality that is inherent to other species; each vessel of the Flood works tirelessly to aid in the propagation of their species.{{Ref/Reuse|bestiarum}}


=== Infection Form ===
The Flood presents a highly variable and unconventional threat in combat, as it can infect and mutate dead or captured opponents into a myriad of deadly forms, effectively growing stronger as it weakens enemy forces. They are widely considered to be the greatest threat to the existence of life (or, more accurately, biodiversity) in the [[Milky Way|Milky Way galaxy]].
{{Main|Infection Form}}
[[Image:Flood3.jpg|200px|left]]
The Flood exists as multiple organisms but the largest it can get without requiring extra bio-mass is the [[Infection Form]]. This form, while not immediately recognisable as a threat from a distance or on its own is dangerous nonetheless. In large swarms this form compensates for its fragility with sheer weight of numbers. Large groups can rapidly overwhelm much larger creatures.  


They are able to infect any sentient organism with sufficient biomass to support them. By latching onto it's target using its leg-like tendrils and using a sharp spine to penetrate deep into the body as far as the spinal cord  the Infection form begins to release spores. First, it paralyzes the host's body. Once this is accomplished, it rewrites the neural pathways of the victim's brain with its tendrils, forcing a resonant frequency match between its neural signals, and the host's. When this is done, it starts to edge its way into the body, slowly moving organs out of the way so it can nest itself within the chest cavity of its host. If a match cannot be obtained, the host is left alone and cannot be used. After forcing a match, it begins releasing spores that mutate the host body with Flood DNA. These spores begin to alter the host, using its calcium stores to create additional flood super-cell biomass, therefore changing the genetic structure of the host's anatomy, mutating its physiology almost beyond recognition: replacing limbs with tentacles for example.
==History==
===Origins===
{{Quote|Our urge to create is immutable; we must create. But the beings we create shall never again reach out in strength against us. All that is created will suffer. All will be born in suffering, endless grayness shall be their lot. All creation will tailor to failure and pain, that never again shall the offspring of the eternal Fount rise up against their creators. Listen to the silence. Ten million years of deep silence. And now, whimpers and cries; not of birth. That is what we bring: a great crushing weight to press down youth and hope. No more will. No more freedom. Nothing new but agonizing death and never good shall come of it. We are the last of those who gave you breath and form, millions of years ago. We are the last of those your kind defied and ruthlessly destroyed. We are the last Precursors. And now we are ''legion''.|The [[Gravemind]] to [[Catalog]] in the endgame of the [[Forerunner-Flood war]].{{Ref/Reuse|precursors}}}}
Over [[10,000,000 BCE|ten million years before]] present day, a civilization of supremely advanced beings known as the [[Precursor]]s thrived in the cosmos. Having assumed the [[Mantle]] - the responsibility for the guardianship of all life in the galaxy - they seeded numerous worlds with life, bringing forth a wide range of [[Sentience|sentient]] species across the galaxy. Under their guardianship, numerous intelligent species developed and flourished, including the [[Forerunner]]s and [[human]]ity. The Precursors would eventually choose humanity to be the rightful successors to their Mantle, while judging the Forerunners unworthy. Refusing to accept their creators' judgment, the Forerunners staged a [[Forerunner-Precursor war|massive rebellion]] against the Precursors.{{Ref/Reuse|precursors}}


Over extended periods of time the Flood infection bacteria seems to become less powerful. These weaknesses allow for the host to control their own thoughts and actions to a degree. As the case was with PFC [[Jenkins]].<ref>[[Halo: The Flood]]</ref>
The ensuing conflict ended with the Forerunners successfully wiping out all but a few Precursors.{{Ref/Novel|Pri|Page=335-336}} The few Precursors that escaped from (or were spared by) the Forerunners either went into suspended animation or transmuted themselves into dust that would regenerate into their past forms at a later time. However, over time the dust became defective, creating sickness, disease and biological mutations in other organisms that came into contact with it. With this new form — the earliest stage of the Flood — the Precursors vowed that none of their creations would rise against them again.{{Ref/Reuse|precursors}} A significantly mutated Precursor, one that would later be known as the last surviving one of its kind,{{Ref/Book|HM|Page=9}} was sealed in a Precursor [[stasis capsule]] and placed in a small, ravaged planetoid at the galaxy's edge around [[9,000,000 BCE]].{{Ref/Novel|Sil|Page=181-182}} This being, later known as the [[Primordial]], would eventually reveal the nature of the Flood to those willing to know the truth.{{Ref/Reuse|precursors}}


=== Combat Form ===
===First outbreak===
[[Image:Warrior Form Covie2.png|200px|right]]
====Antecedents====
[[Image:Warrior Form Human2.png|200px|right]]
The Flood, as it is known in modern times, first appeared in the galaxy some time prior to [[107,445 BCE]], after [[Ancient humanity|ancient humans]] came into contact with the Precursor dust. This fine, desiccated powder had been stored in millions of glass cylinders placed aboard automated starships,{{Ref/Novel|Id=origin|Cry|Page=268-270}} bound toward the margins of the Milky Way galaxy from the direction of the [[Path Kethona|Large Magellanic Cloud]].{{Ref/Novel|Id=sil35|Sil|Page=35}} [[Forthencho]] came to believe that the [[Primordial]], despite its hibernating state at the time, may have been responsible for remotely triggering the movement of these ships.{{Ref/Novel|Pri|Page=226}} However, over a span of ten million years, the substance had slowly deteriorated, along with the ships themselves failing.{{Ref/Reuse|origin}}
{{Main|Combat Form}}
After an infection form has obtained a frequency match for the host's neural system, it begins to rewrite the very DNA of the body it has infected. Once fully transformed, the Flood infection is in total control of the body. Unfortunately for the host, he or she stays alive throughout the transformation process and is even conscious of his environment as the Flood takes over the body. Once completely infected, the Flood becomes what is known as a Combat form. Any body of sufficient size, capable of carrying a weapon or exhibits a possible use in combat is transformed into one of these monstrosities. The Combat form grows powerful, whip-like tendrils on its arms, and will use them as close combat [[weapons]], or if it loses the ranged weapon it carries.


The Flood infection is capable of not only controlling the host's body, but of also tapping into and utilising its memories. Most ofen for the purpose of finding relevant information about the use of weapons and equipment. After the Combat form has suffered a sufficient level of punishment as to be no longer effective, it mutates into a [[carrier form]]. A biological incubator for more [[infection forms]]. Eventually a combat form will mutate into a carrier, regardless of damage, to perpetuate the Flood species.
The crashed remains of these ships were discovered by ancient humans on both inhabited and deserted worlds near the border of [[intergalactic space]], a region of the galaxy to which humans expanded early on in their history. Inside the ships, humanity found the capsules filled with powder, which their tests told them consisted of inert, short-chain organic molecules of unknown purpose.{{Ref/Reuse|origin}} Humanity studied the material and deemed it to be harmless, but noted that it had psychotropic effects on lower animals. Small dosages were [[Drugs|administered]] to [[Pheru]], popular domestic animals among humans and San'Shyuum, which resulted in more docile behavior.{{Ref/Novel|Id=cryptum131|Cry|Page=131}} Unbeknownst to the humans or San'Shyuum at the time, the powder was altering the genetic code of the Pheru, manifesting as improved behavior while slowly changing other aspects of their genome.{{Ref/Reuse|origin}}


[[Combat forms]] can take extensive damage before being neutralized. Arms, legs, even heads can be totally blown away from their bodies before they are incapacitated. The only known weakness of the combat form is an area on the upper torso, where sensory feelers jut forth. This is where the Infection form has nestled, shots to this area will kill the Infection form within, effectively killing the Flood Combat form.
====Emergence====
[[Image:Flood_Outline.jpg|left|thumb|200px|Combat Form's outline on Halo3.com]]
After the humans had exposed the Pheru to the powder for several centuries, more disturbing changes appeared. Pheru treated with the powder began exhibiting furry growths and fleshy protrusions, which other Pheru were compelled to consume. Shortly afterward, the Pheru began suffering widespread abortions and genetic deformities, and many were euthanized or released into the wild. Soon afterward, the humans and San'Shyuum who had been in the presence of the altered Pheru began to exhibit some symptoms of the powder-induced "disease". Affected on a psychological level by the strange illness, infected victims began to consume the sick Pheru, and anything they touched became infected while parts of themselves that were discarded, such as tissue and limbs, also became vectors for the disease. The afflicted then moved on to cannibalism and sacrifice of the infected, inducing a sacrificial individual to grow to prodigious size and then forcing uninfected individuals to consume them while they were still alive. During this time, the infection passed from human to San'Shyuum and human to human, but rarely San'Shyuum to human. The infected humans and San'Shyuum proceeded to spread the disease into other systems, which furthered the scope of the Flood outbreak.{{Ref/Reuse|origin}}
Most often [[Humans]] and [[Elites]] are used as host bodies for the Combat form, only strong host bodies are taken over for such a purpose. Weaker species are used for Carrier forms - namely [[Grunts]] and [[Jackals]].
 
{{Clear}}
At this point, the Flood infestation, which had previously been predominantly a mind and behavior-altering affliction with only relatively minor physical changes, began to reshape its victims into a variety of forms designed to gather victims, engage in combat, and to rapidly consume useful biomass and other resources.{{Ref/Reuse|origin}}
 
====Spread and withdrawal====
The infection quickly spread through both human and Forerunner space. Attempts to quarantine infected worlds and systems failed, leading to the Flood soon infecting hundreds of worlds in fifteen systems.{{Ref/Reuse|origin}} In an effort to halt the Flood's continuing spread, human fleets began destroying Flood-infested Forerunner ships and cleansing Forerunner planets where Flood infestations were detected. [[Forthencho]], the commander of the human fleets, chose not to warn the Forerunners in advance of these actions to prevent giving the Flood time to expand.{{Ref/Game|Id=terminals4|H4|Detail=[[Terminal (Halo 4)|Terminals]]}} In turn, many Forerunners believed that the Flood was a fiction meant to hide humanity's expansionist aims. Ultimately, the Forerunners interpreted humanity's tactics as acts of aggression, leading to the [[Human-Forerunner wars]].{{Ref/Reuse|origin}}
 
With humanity severely weakened from the Flood infection and now fighting a two-front war, the Forerunners were surprised at the ease of their earliest victories against the humans. However, apparently without warning, the Flood began to retreat from its assault on humans. Forerunner ships found healthy human worlds within sectors of Flood infestation, leading Forerunners to conclude that humanity had developed a natural resistance, immunity or cure to Flood infection.{{Ref/Novel|Id=sil34|Sil|Page=34-37}} However, no immunity or cure to the Flood existed; the Flood had chosen to stop infecting humans.{{Ref/Novel|Id=p364|Pri|Page=364-365}} The [[Ur-Didact|Didact]] later speculated that the Flood's intent was to prevent humanity's extinction at the hands of the Forerunners by making it appear as if humans held a solution to Flood infection.{{Ref/Novel|Sil|Page=170}}
 
At this time, the Forerunners also first came into contact with the Flood. The parasite devastated many of their fleets, before its deliberate recession from the galaxy tipped the scales in the Forerunners' favor, allowing them to sterilize any Flood infestation they came across in several thousands of worlds across the galactic border.{{Ref/Reuse|sil34}} While recognizing the legitimate concern of a Flood resurgence in the future, the Forerunners had difficulty extracting any intact specimens of the parasite for study; approaching the threat with appropriate caution, the [[Warrior-Servant]]s directly cauterized any active infestations they encountered. From the scattered remains recovered for analysis, the Forerunners discovered that AIs that had been in contact with the Flood had been corrupted beyond repair by what the Forerunners would come to know as the [[logic plague]], essentially the data equivalent to Flood infection.{{Ref/Reuse|sil34}} No fully intact specimen of the Flood could be found among the Pheru, humans, or San'Shyuum, as they had methodically destroyed any sign of infection, including all the original carrier vessels that they had discovered.{{Ref/Reuse|c272}} Any remaining Flood were believed to have fled the galaxy aboard infected ships,{{Ref/Novel|Id=c272|Cry|Page=272}} and the Flood was not seen again for nearly ten thousand years.
 
While they managed to regain some of their strength once the Flood began to recede, humanity still stood against the Forerunners in the later stages of the conflict. Their campaign against the Flood had worn their resources thin, while the Forerunners led by the [[Ur-Didact|Didact]] had managed to entrench their forces in advantageous positions which proved too powerful for humans to hold against.{{Ref/Reuse|c272}} System by system, the humans fell back to their capital, [[Charum Hakkor]], where they were finally defeated. After losing the conflict, the Forerunners devolved humanity and exiled them to [[Earth|their homeworld]] while erasing all signs of human achievement from the galaxy.
 
===Interim===
Due to the lack of evidence, most Forerunners were unaware of the existence or of the true nature of the Flood, and many dismissed it as a simple excuse for humanity's aggressive expansion before and during the initial arrival of the Flood. However, several high-ranking Forerunners were fully informed of the threat, and in secret began developing countermeasures should the Flood ever return. This became the source of a major political strife, with the [[Builder]]s proposing the construction of an array of superweapons - [[Halo Array|Halos]] - while the [[Warrior-Servant]]s led by the [[Ur-Didact|Didact]] championed a more conventional strategy of carrying out precision military campaigns from planet-sized fortresses known as [[shield world]]s. The Didact eventually lost to the Builders and the entire [[Warrior-Servant]] rate was marginalized, with their fleets and forces disbanded or forced to submit under Builder control. This proved to have been a grave mistake, as the Forerunners found themselves ill-prepared to fight off the Flood after the parasite returned to the galaxy.
 
Realizing that the Flood stood a significant chance of wiping out all sentient life and the prospect of using the Halos to stop them, the [[Lifeworker]] race of the Forerunners began a "[[Conservation Measure]]", researching the Flood while cataloging and preserving the galaxy's species aboard a number of facilities related to the Halo Array, mainly an extragalactic installation known as [[greater Ark|the Ark]].
 
===Forerunner-Flood war===
{{main|Forerunner-Flood war}}
[[File:Anniversary, The Flood.jpg|thumb|300px|A Flood-infested Forerunner city, moments before the [[Halo Array]]'s activation.]]
{{quote|It fed on intelligent life and, in doing so, became ever more intelligent itself. The Flood was unique; it used [[Forerunners|their]] very strength against them.|Cortana on the Forerunners' war against the Flood.}}
The Flood reappeared as a viable threat in the galaxy when they were first encountered by the Forerunners on the planet [[Seaward]]. Initially, the Forerunners severely underestimated the potential threat that this parasitic lifeform posed, and used tactics more suited to disease control than actual warfare. By the time the infection had begun to spread, the Flood had become a much deadlier and more intelligent foe. While at first the Flood had demonstrated a lack of even rudimentary strategic ability, the parasite quickly began to amass biomass and create [[Gravemind]]s, Flood forms that embodied a collective intelligence for the Flood species, and that could coordinate the Flood swarms in attacks against the Forerunner fleets using all Flood forms within the vast range of their seemingly telepathic control.{{Ref/Reuse|terminals}}
 
A common tactic used by the Flood was to use huge numbers of unarmed civilian vessels to penetrate planetary defense groups, bypassing the Forerunners' orbital [[Forerunner fleet|fleet]]s, and infesting the planet and its populace below. The sheer numbers of Flood forms on the planets overwhelmed Forerunner ground forces. Billions of forms were dedicated to planetary assaults, and every member of the Forerunner population was a potential Flood host. These tactics were demonstrated in the Flood's attacks on numerous worlds, including [[LP 656-38 e]] and [[DM-3-1123b]].{{Ref/Game|Id=terminals|H3|Detail=[[Terminal (Halo 3)|Terminals]]}} In desperation, the Forerunner armada was ordered to commence full [[orbital bombardment|planetary bombardment]] on infested worlds, although at a great cost - many Forerunners were unable to be evacuated before the bombardment commenced, leaving the Forerunner military with shallow victories against the Flood. In events where the naval garrisons were unable to commence bombardment, major Forerunner population centers were seen to activate localized weapons of mass destruction, effectively committing mass suicide, but stopping the growing infestation.{{Ref/Reuse|terminals}}
 
Thereafter, the Forerunners realized that conventional naval tactics were unable to stem the growing tide of the Flood, and decided to pin their hopes of defeating the Flood on more extreme measures. While the Forerunners initially deployed robotic [[Sentinel]] drones to fight against and contain the Flood, almost immediately afterwards, the [[Forerunner Fleet Command]] contemplated "Premature stellar collapse": using naval battle groups to send stars into [[supernova]]e that would engulf their planetary systems and prevent any possibility of Flood infection and destroy all Flood present in the system.{{Ref/Reuse|terminals}} As a further setback to the Forerunners, an extended dialog with the Primordial provoked [[Mendicant Bias]], the [[Contender-class artificial intelligence|Contender-class AI]] in charge of the Forerunner defense, to succumb to the Flood's [[logic plague]] and turn its vast resources against its masters. While the Primordial's physical body was eventually destroyed, it's consciousness was either consumed or transferred to the Gravemind, granting the Flood previously unprecedented intelligence and power.{{Ref/Book|HM|Page=32}}
 
====Final decade====
The Flood grew exponentially, spreading from system to system with the intent to infect the Forerunner population, not to wage war with Forerunner battle groups which were poorly adapted to such strategy. As the Flood spread further, entire planetary biospheres were converted to Flood biomass. These planet-sized [[key mind]]s, locuses of Gravemind intelligence that began to outcompete the Forerunners through raw computational might alone.{{Ref/Novel|Id=key mind|Sil|Page=186-187}}<ref name="deluge">[https://www.halowaypoint.com/en-us/news/deluge-delights ''Halo Waypoint'': "Canon Fodder - Deluge Delights"]</ref> The logic plague continued to take on more adaptive forms, defying all defenses the Forerunners devised and eventually spreading itself between AIs and data networks. As the Forerunners relied significantly on AI automation in combat, this severely hindered Forerunner military efforts against the Flood. Adding to these difficulties was the disturbing revelation that the Flood was capable of manipulating [[Precursor]] technology: towards the close of the war, the Flood began to appropriate Precursor megastructures such as the [[star road]]s, wielding the previously-inert strands to pummel Forerunner planets and naval forces. This ability was a result of the Flood's Precursor origins; being corrupted Precursors, they could tap into [[neural physics]] and activate Precursor artifacts. The proliferation of high-level key mind nodes also enabled the Flood to use the Precursors' neural physics-based superluminal transit, which in turn encumbered Forerunner slipspace travel. Towards the end of the infestation, Forerunners noted that space itself seemed to become hostile and unpleasant from mere perception, an artefact of Flood-based Neural Physics. {{Ref/Reuse|key mind}}
 
====The Halos' firing====
{{Main|Great Purification}}
The Forerunners employed countless measures to contain the Flood outbreak, all of which failed. Finally, the Forerunners were forced to a Pyrrhic solution: using an array of seven ring-shaped megastructures collectively known as the [[Halo Array]] as superweapons, the Forerunners would purge the galaxy of all sentient life. Because the Flood were parasitic, and their survival was directly linked to the presence of potential hosts (and in particular hosts capable of sentience) the Forerunners reasoned that eliminating all potential Flood hosts would render the parasite unable to grow and spread, and eventually cause it to starve to death. Additionally, as a result of the Master Builder's test of [[Installation 07|a Halo]] at [[Charum Hakkor]], it was found that the Halo Effect unintentionally caused the complete destruction of artefacts of Neural Physics. This was likely a byproduct of how the Halo Effect was designed to destroy any and all forms of neurological structure subjected to it.{{Ref/Novel|Id=cryptum 293|Cry|Page=293-294}} The decision led to political battles and even a civil war, but after exhausting every countermeasure, the Forerunners activated the Halos in [[97,445 BCE]].
 
As a result of the firing of the seven Halo installations, the surviving Flood were successfully contained and eventually died out. The only surviving specimens were contained in state-of-the-art, high-security Forerunner research facilities, such as the Halo installations, certain Shield Worlds, and the [[Threshold gas mine]]. All Precursor artifacts and structures, composed and maintained entirely using Neural Physics were also destroyed and left no trace.{{Ref/Novel|Sil|Page=326}} Many sentient species throughout the galaxy (including humanity) survived the firing of the Halo Array, having been safely relocated beyond the galaxy by the Forerunners on [[Installation 00|the Ark]].
 
During their expansion throughout the Orion Arm, the [[Covenant]] would find and occasionally release the Flood, employing [[Zealot]]s and their [[Temple Wraith]]s to contain the outbreaks.{{Ref/Game|H5|List of REQ cards|REQ card|Detail=[[Temple Wraith]] description}}{{Ref/Book|Enc22|Page=224}}
 
===Outbreak on Trove===
{{main|Battle of Trove}}
Thousands of years after the end of the Forerunner-Flood war, the human ship {{UNSCShip|Spirit of Fire}} jumped from [[Arcadia]] to follow the signal of [[Ellen Anders|Professor Anders]], who had been captured by the Covenant. They were led to an unknown planet, which happened to be [[Trove|a Forerunner shield world]]. Containment measures on this planet had failed catastrophically, and the entire surface of the megastructure was choked with Flood biomass. Ground forces were deployed to investigate the presence of unknown hostiles sighted by the ship's radar. After fighting their way through Covenant forces, [[John Forge|Sergeant Forge]] and several Marine squads were attacked by the Flood.
 
After successfully defeating the Flood forces, the UNSC forces created a [[firebase]] in the area that Anders' signal appeared to be coming from. The humans eventually tracked the signal to a massive [[Proto-Gravemind]] attached to several [[Flood colony|Flood colonies]]. The Proto-Gravemind had duplicated Anders' signal, possibly in hopes of infecting the humans and spreading beyond Etran Harborage. It was discovered that killing a Flood colony weakened the Proto-Gravemind and [[SPARTAN-II]] [[Red Team]], with help from the firebase, destroyed the Proto-Gravemind.
 
The Flood responded by launching a massive attack on the ''Spirit of Fire's'' [[Alpha Platoon|Alpha]], [[Bravo Platoon|Bravo]] and [[Charlie Platoon]]s to avoid them returning to the ship. Despite massive Flood resistance, forces from the ''Spirit of Fire'' were able to rescue the platoons before their firebase was overrun by the Flood.
 
With the Proto-Gravemind dead, the smart-[[AI]] [[Serina]] was able to reacquire Anders' actual signal, which led to an ocean under which a docking port leading to the interior shield world was located. During the ''Spirit of Fire's'' descent, the outer surface of the ship was infested by the Flood. With the aid of Sentinels and "cleansing rings" controlled by the installation's AI, the ''Spirit of Fire'' crew were able to cleanse the ship and proceed into the shield world.
 
Within, the UNSC forces discovered that the Flood had managed to infest the interior of the shield world as well, though not to as massive of a degree as the surface planet itself. While rescuing Anders, Forge had to save her from three [[pod infector]]s and ran into some trouble with Flood forces on his following missions, though not to the massive extent they had been seen on the exterior planet.
 
In order to prevent the Covenant from getting their hands on Etran Harborage's fleet of Forerunner {{Class|Sojourner|dreadnought}}s, a plan was created to use the reactor from the ''Spirit of Fire's'' [[Shaw-Fujikawa Translight Engine]] to induce the shield world's artificial star to go [[supernova]]. The plan was a success and Etran Harborage was destroyed. The explosion obliterated the Flood on the planet, along with the Covenant forces and the Forerunner fleet.{{Ref/Game|HW}}
 
====Brief recurrence====
{{main|Outbreak on the UNSC Spirit of Fire}}
On January 1, [[2537]], six years after the events on [[Trove]], a Flood outbreak was initiated aboard the {{UNSCShip|Spirit of Fire}} by a [[Pod infector]] that had managed to remain hidden aboard the ship. As the Flood threatened to breach quarantine, Serina awoke [[Jerome-092]] who, with the aid of Professor [[Ellen Anders]], was forced to exterminate all infected crew members, ensuring the survival of the ''Spirit of Fire'' and its remaining crew. All traces of the Flood parasite aboard the ''Spirit of Fire'' were erased.{{Ref/Comic|TFS|Detail=''[[Something Has Happened]]''}}
 
===Revival of the Flood===
{{Main|Battle of Installation 04|Outbreak at the Containment Facility}}
[[File:Anniversary, flood outbreak.jpg|thumb|300px|[[Jacob Keyes]] and his Marine squad are overwhelmed by the Flood during their expedition into a [[Flood containment facility]].]]
{{Quote|The only way to stop the Flood is to starve them to death.|[[Cortana]] to [[John-117]] and [[343 Guilty Spark]].}}
 
Although the first firing of the Halo Array starved the Flood that were at large in the Milky Way Galaxy, Flood specimens were kept in stasis within installations around the galaxy, including [[Installation 04]] and various Forerunner research facilities. This would prove to be a grave mistake, as both humanity and the Covenant stumbled upon  [[Installation 04|one]] of the abandoned Halo rings thousands of years after the firing of the Array. The UNSC [[Artificial intelligence|AI]] [[Cortana]], aboard the {{UNSCShip|Pillar of Autumn}}, made what was presumed to be a "blind" jump through Slipspace (in reality, she used translated co-ordinates from a [[Sigma Octanus IV artifact|Forerunner artifact]] found on [[Sigma Octanus IV]]) to evade Covenant pursuers. This led both groups to Installation 04, and in turn, the Flood that were kept there in stasis. After the crash of the ''Pillar of Autumn'' on the Halo ring, piloted by [[Captain (Navy)|Captain]] [[Jacob Keyes]], the surviving crewmembers dug in for what they believed would be a long and brutal guerrilla campaign against the Covenant forces on the ring.
 
Meanwhile, the pursuing Covenant cruisers, among them the ''[[Truth and Reconciliation]]'', amassed their ground forces on the surface, where they soon discovered an underground Forerunner [[Flood containment facility]] in the swamps of the ring. Due to the secure nature of the facility, the Covenant came to use it as a fortified base of operations and as a storage area for weapons and ammunition, oblivious to its true nature and purpose.
 
At some point, either through a malfunction in containment equipment or a blunder on the Covenant's part, the captive Flood forms within the facility's stasis chambers were released and attacked the Covenant forces inside. The Covenant had at least a rudimentary knowledge of what the Flood were capable of, and promptly locked the facility down and evacuated all personnel who survived the incident.
 
Keyes and his team were unaware of these events, and proceeded to move deep into the facility, ultimately coming across one of the unopened Flood containment chambers. Believing it was a weapons cache of some sort, the team unlocked the door, and were attacked and assimilated by the Pod infectors inside. Upon being informed of Keyes' disappearance, John-117 was deployed to Captain Keyes' last known location on the drop ship ''[[Echo 419]]'', proceeding into the facility, only to find out that most of the Marines he had been assigned to rescue, including the Captain himself, had succumbed to the Flood.
 
These events brought knowledge of the Flood to the UNSC. In a war of losses, this was another grim revelation for humanity.{{Ref/Game|Id=CE|CE}}
 
====Voyage of the ''Infinite Succor''====
{{Main|Skirmish on Infinite Succor}}
Shortly after their escape from the containment facility, Flood forces took control of a nearby crashed Spirit dropship, the ''[[Brilliant Gift]]'' and crashed it aboard the Covenant {{Pattern|Mjern|agricultural support ship}} ''[[Infinite Succor]]''. The Flood quickly spread throughout the ship, infecting its crew and the animals from the vessel's hunting preserve. The Flood began building a [[Proto-Gravemind]], intending to use the ''Infinite Succor'' to flee the [[Soell system]] and begin infecting other planets. The ship's Legate, the [[Minister of Etiology]], locked out the ''Infinite Succor's'' primary systems while one of the crew's [[Unggoy]] managed to send out a distress call. Suspecting human involvement, particularly that of [[John-117|the Demon]], [[Supreme Commander]] [[Thel 'Vadam]]ee sent a [[Covenant Special Operations]] team led by [[Rtas 'Vadum]]ee to retake the ship and eliminate the Demon if he were onboard. Quickly uncovering the true threat, Rtas programed the ship to make a [[slipspace]] jump into the system's star which would destroy the ''Infinite Succor'' and the Flood infestation onboard. Though Rtas succeeded, he was the only survivor.{{Ref/Comic|HGN|Detail=''[[The Last Voyage of the Infinite Succor]]''}}
 
====Assault====
[[File:Annivesary, conflict.jpg|thumb|400px|The Flood engages both [[UNSC]] and [[Covenant]] in their drive to escape [[Installation 04]].]]
After the Flood's escape, the Covenant forces across [[Installation 04]] were overwhelmed by the Flood's assault. While the Covenant were able to hold the line by using vehicular and air support, most of the weaker Covenant soldiers such as [[Unggoy]] and [[Kig-Yar]] succumbed easily to the Flood, and the few Sangheili who remained were outnumbered and thus infected. The UNSC, having a much smaller presence on the installation, found out about the Flood's might a short while later, when an [[Orbital Drop Shock Troopers|ODST]] force engaged with the Covenant near [[Alpha Base (Installation 04)|Alpha Base]], was ambushed by the Flood and suffered heavy casualties. However, one of the attacking combat forms, [[Private First Class|PFC]] [[Wallace A. Jenkins]], who was one of the Marines in the original expeditionary mission to the supposed arms cache, was infected by a weak and elderly Pod infector whose powers of infection had been dulled by centuries of hibernation, making Jenkins' transformation into a combat form incomplete. Jenkins was still able to resist the pervasive Flood entity in his mind, and although incapable of articulate speech and with a physiologically devastated body, Jenkins was still able to betray the Flood through hand gestures during an interrogation with [[Major (UNSC)|Major]] [[Antonio Silva]], informing the UNSC Marine battalion that the Flood planned to assault the UNSC base from a series of underground catacombs underneath their current base of operations. In response to this intelligence, the UNSC moved to secure them, and prevented the Flood from staging another attack.{{Ref/Reuse|TF}}
 
During these developments, the ring's Sentinels failed to effectively contain the Flood, and because of this, the Covenant and the UNSC sustained heavy losses. The Flood occupied areas of the ring formerly held by the Covenant, although Covenant and Flood forces continued to wage war on the massive ice plains of the ring. Although the Covenant had the benefit of [[Shade]] stationary [[plasma]] turrets and vehicular/aerial support, the Flood had captured both UNSC and Covenant weapons and because of this had significant numerical superiority.
 
One of the UNSC personnel captured by the Flood was [[Captain (Navy)|Captain]] [[Jacob Keyes]], a brilliant naval strategist of the [[UNSC Navy]]. When he was infected, the Flood sensed that he held knowledge that would be especially useful to the Flood. Keyes knew the location of [[Earth]], the birthplace and major stronghold of the human race, a planet with tremendous possible assimilation opportunities for the Flood. Instead of becoming a combat form, he was merged with at least four other victims into a massive, engorged proto-Gravemind aboard the {{Pattern|Ket|battlecruiser}} ''[[Truth and Reconciliation]]'', a Covenant warship the Flood had taken over. While he was being incorporated into the proto-Gravemind, the Flood intelligence began to dig through his mind, searching his memories for the location of Earth. Captain Keyes successfully withheld information about Earth by constantly accessing information available on his [[command neural interface]] such as his name, rank, and serial number, until John-117 arrived. However, his arrival was too late, for Keyes was already dead, completely consumed by the [[Proto-Gravemind]]. It was at that time that John-117 was forced to punch a hole into Keyes' skull to obtain the Captain's CNI transponder.{{Ref/Level|CE|[[Keyes]]}}
 
The ''Truth and Reconciliation'' was a major point of contention between the [[Covenant Special Operations]] units and the Flood. The Flood had decimated most of the Covenant crew and garrison on board, leaving only isolated pockets of resistance, mostly small numbers of terrified Unggoy and a few [[Sangheili]] aboard the vessel. However, below the ''Truth and Reconciliation'', in a series of rocky canyons, the Covenant were much stronger in force, with large numbers of Sangheili augmented by Unggoy, Kig-yar, and [[Mgalekgolo]], fighting off the Flood. The Covenant [[Council of Masters]] in charge of operations on Installation 04, however, were terrified by the Flood presence. Because the Covenant battlecruiser had been damaged in a previous space battle with the {{UNSCShip|Pillar of Autumn}}, it had been grounded, awaiting repairs to its underbelly and plasma carrying conduits, for at least two days. After the Flood were discovered to be aboard the battlecruiser, the Covenant military leadership sent a Special Operations strike force to the ''Truth and Reconciliation'' to commence immediate emergency repairs, prepare it for lift-off into space and neutralize the Flood. These efforts, however, failed, with the Flood eventually retaking the vessel.
 
Later, the remaining UNSC Marine forces on the ring staged an assault on the Flood-held ''Truth and Reconciliation'', and managed to neutralize most Flood forms on the cruiser in a swift assault, and prepared the battlecruiser for take-off with hastily completed emergency repairs to escape from the ring and return to Earth. However, there was tremendous risk if the Flood-infected vessel traveled to Earth, if there was even one Flood carrier form aboard, "''Earth could fall,''" according to UNSC [[Artificial intelligence|AI]] [[Wellsley]], because of the Flood's exponential parasitic potential. Against orders, Marine First Lieutenant [[Melissa McKay]] destroyed the warship with whatever Flood forms were still aboard, killing hundreds of UNSC personnel, including an infected Jenkins, but saving Earth and possibly billions of lives from the Flood.{{Ref/Novel|Id=TF|TF}}
 
====Defeat====
[[File:PoADestructionCEA.jpg|thumb|300px|The ''Pillar of Autumn'' - exploding, with its fusion reactor becoming destabilized, destroying both the ship and the ringworld, to prevent the Installation from being fired and taking the threat of the Flood with it.]]
The Flood were interested in seizing space-capable vessels, in the hope of spreading the infection away from Installation 04 and across other systems; they attacked the {{UNSCShip|Pillar of Autumn}} and cruiser ''[[Truth and Reconciliation]]'' for this reason. After the UNSC cruiser's crash-landing onto the ring, and after engaging over fifteen enemy warships previously to landing, it had been taken by the Covenant, although a UNSC strike team had managed to temporarily retake the bridge. On the third and final day of the Battle of Installation 04, however, the Covenant were able to gain control of the crashed warship while one of many Covenant patrols around the exterior of the ''Autumn'' was ambushed by the Flood and infected; upon the patrol's return to the vessel, the Covenant garrison was attacked by the infected Elite combat forms of the patrol group. The combat forms were able to scatter before they could all be destroyed, and a steady trickle of Flood continued to enter the ''Pillar of Autumn'' through vents and openings in the hull, waging a tedious guerrilla battle against the Covenant security force on board. On another occasion, the Flood attempted to enter through cargo containers but were foiled due to the quick thinking of the [[Unggoy]] [[Yayap]].
 
Eventually, a force of [[Sentinel Aggressor|Sentinels]] came on board, overwhelming the dwindling Covenant forces as the Flood began to attack the warship in full force, in the end almost completely obliterating the Covenant presence, and severely limiting the Sentinels' numbers on board. Despite their best efforts and enormous numbers, the Flood, Covenant, and Sentinels were unable to stop [[John-117]] from detonating the wrecked warship's reactors, and escaping in a [[GA-TL1 Longsword|Longsword interceptor]]. The resulting thermonuclear detonation from the destabilized fusion reactor cores severed a portion of the ring world, ultimately destroying it and its capability to annihilate all sentient life, and simultaneously killed all remaining Flood on Installation 04, along with any other life forms that were still present on the ring.{{Ref/Reuse|TF}}{{Ref/Reuse|CE}}
 
====Outbreak on the ''Mona Lisa''====
{{Main|Skirmish on Mona Lisa}}
After the destruction of Installation 04, in October 2552, a human [[prison ship]], the ''[[Mona Lisa]]'', arrived in the Soell system and managed to retrieve surviving specimens of the Flood for study from the remains of Installation 04. They began to test the parasite on both human and Covenant prisoners, unaware of its lethality. Soon, the infection broke quarantine and the ship was infested by the Flood. The Flood absorbed the crew and prisoners, then began to build a proto-Gravemind in its engineering section. The infestation ended when Sergeant [[Zhao Heng Lopez]] and her marines from the {{UNSCShip|Red Horse}} were sent to investigate the ship after retrieving a [[John Doe|survivor]] from the ship. The Flood absorbed many of her marines, leaving her and a single [[Henry|Sangheili]] to fight for a single-occupant escape pod. The outbreak was most likely contained when the ''Red Horse'' destroyed the ''Mona Lisa'' using a [[Shiva-class nuclear missile]].{{Ref/Novel|Id=mona lisa|Evo|Detail=''[[The Mona Lisa]]''}}
 
===Threshold gas mine===
{{Main|Raid on the Threshold gas mine}}
[[File:H2A Tadpole3.png|thumb|left|250px|Dormant Flood in receptacles housed in the gas mine.]]
The Flood were also kept in a [[Forerunner]]-constructed [[Threshold gas mine|gas mining facility]] suspended over [[Threshold]], within at least one of the three arms of the station. A small [[Covenant Special Operations|Covenant special forces]] strike team, led by [[Special Operations Commander]] [[Rtas 'Vadumee]] and the [[Arbiter]] [[Thel 'Vadam]]ee, were dispatched by the [[High Prophets]] to silence [[Sesa 'Refumee]], the leader of a [[Sesa 'Refumee's heretic faction|heretic faction]], who was stationed within the mine. The heretics released the dormant Flood within the station in an attempt to halt the progress of the Spec-Ops team through the station.
 
The heretics' plan backfired as the Flood killed and infected many of their forces while failing to stop the Arbiter and his team. The Flood broke out of the station's arm and spread to the central core of the station, where they overwhelmed the heretics stationed there. Eventually, even the station's Sentinel forces were completely obliterated as the Flood spread to the upper levels of the station. The Flood were unable to escape the station, however, as the Arbiter managed to sever the cable attaching it to the Forerunner support structure far above, sending the gas mine plummeting to Threshold's liquid hydrogen core, destroying it.{{Ref/Level|H2|[[The Oracle]]}}
 
===Battle of Installation 05===
[[File:H2A-Miranda.jpg|thumb|250px|The Flood were in search of the [[Activation Index]], a device which would deprive UNSC and Covenant forces from the privilege of assured destruction of the parasite.]]
{{main|Battle of Installation 05}}
On Installation 05, the Flood had been in a continual battle with the Sentinels after the initial firing of the Halos 100,000 years prior. Again, a secure facility was breached and a Flood form managed to escape, thus beginning the battle with Sentinels. While the infestation had been contained to a quarantine zone established outside the Installation's Library, and presumably was severely weakened due to a lack of fresh host bodies, the Flood had managed to keep a Gravemind form alive deep within the cavernous tunnels of the ring. With the arrival of the Covenant and pursuing UNSC forces, the Flood was able to grow its forces considerably and redouble its assault on the Installation's security forces.
 
The Flood, despite a significant vehicular and infantry presence in the Quarantine Zone surrounding the Library, were unable to deny access to the Covenant and the UNSC. [[Miranda Keyes]] managed to slip the ''[[UNSC In Amber Clad|In Amber Clad]]'' through the Forerunner Enforcer patrols, while the Covenant launched a ground attack, led by Thel 'Vadamee and involving a small team of Spec Ops Elites, using two [[anti-gravity gondola]]s to get to the index while attempting to control the gondolas and keep it from slipping into Flood or Sentinel hands. Later, the ''In Amber Clad'' was captured by the Flood.{{Ref/Game|Id=H2|H2}}
 
===Fall of High Charity===
{{Main|Fall of High Charity}}
[[John-117]], who had arrived with Commander [[Miranda Keyes]] and Sergeant [[Avery Junior Johnson|Johnson]], met [[Thel 'Vadam]]ee in the clutches of the [[Gravemind]], the central controlling intelligence of the Flood hive, created from the combined intellect of tens of thousands of assimilated corpses. The Gravemind attempted to convince Thel, against his own beliefs, and through the use of the captured [[Monitor]], [[2401 Penitent Tangent]], and the semi-revived and consumed [[Prophet of Regret]] to not activate the ring. Even though Thel was still unswayed, although in question of his religion, the Gravemind sent him and John-117 to search for the Activation Index to keep the Covenant from "turning" the "key," transporting John-117 to [[High Charity]], and Thel to Delta Halo's [[Control Room]].{{Ref/Level|Id=Gravemind|H2|[[Gravemind (level)|Gravemind]]}}
 
Taking advantage of the distraction that John-117's presence caused, as well as the ensuing [[Great Schism]], the Gravemind took the opportunity to deliver itself, as well as a multitude of Flood underlings, onto the {{UNSCShip|In Amber Clad}} and then slipspace jumped the UNSC ship directly inside High Charity, whereupon it immediately crashed into the city's superstructure to deliver its infectious cargo, while a wave of Flood-controlled [[D77-TC Pelican|Pelican dropships]] delivered Flood forms all over the city. The ill-equipped and distracted populace was no match for the Flood onslaught, and the Covenant Holy City was quickly overrun, with even the [[Prophet of Mercy]] falling victim to a Pod infector.{{Ref/Reuse|Gravemind}}
 
The Flood soon spread to every district, and Flood spores began to infiltrate and overload the ventilation systems within High Charity; the whole city was becoming terraformed into a gigantic Flood hive. The Gravemind regularly rebutted the words of comfort the Prophet of Truth regularly delivered to the city’s population over the comm system, to sadistically instill more panic and unease. John-117, having successfully stayed in front of the wave of Flood running rampant in the streets, stowed away on the [[Anodyne Spirit|Forerunner Dreadnought]] that was leading [[Truth's fleet]] to Earth. Cortana, who had to stay behind to detonate ''In Amber Clad's'' engines should the situation arise and Installation 05 be fired, was left in the clutches of the Gravemind, who could potentially learn all possible human knowledge from her, including the entire knowledge directly received from Forerunner systems aboard Installation 04.{{Ref/Level|H2|[[High Charity (level)|High Charity]]}}{{Ref/Level|H2|[[The Great Journey]]}}
 
===Earth, the Ark, and the Flood's defeat===
[[File:Infection.jpg|230px|thumb|A UNSC Marine being infected by the Flood.]]
{{main|Outbreak on Earth|Battle of Installation 00}}
 
At the conclusion of the [[Battle of Voi]], with the [[Prophet of Truth]] activating [[the Portal]] and the Jiralhanae fleet escaping to [[Installation 00|the Ark]], a [[Indulgence of Conviction|Flood-infested Covenant cruiser]] appeared out of [[Slipspace]], having penetrated the Elites' blockade of High Charity and crash landed into the city of [[Voi]] in an effort to spread the parasite.{{Ref/Level|Id=The Storm|H3|[[The Storm]]}} In order to prevent the Flood from escaping and infecting the entire planet, Commander [[Miranda Keyes]] ordered John-117 to detonate the cruiser's reactors and search the ship for Cortana, causing the city of Voi to be destroyed and the Flood there eliminated. As John-117 and [[Thel 'Vadam]] fought their way to the ship, the [[Fleet of Retribution|Sangheili]] assault carrier ''[[Shadow of Intent]]'' under the command of [[Rtas 'Vadum]] arrived and deployed Sangheili task force to aid UNSC marines. John-117, with the help of the Sangheili, was able to battle through the Flood forces, including the newly encountered Flood [[Pure Form]]s to reach the crashed cruiser where he found a message from [[Cortana]]. Upon this discovery, [[343 Guilty Spark]] arrived, pledging his assistance to the [[Reclaimer]]s. With the Flood spreading, John-117 boarded a Phantom while the Sangheili went on to glass the infected areas. The remaining [[Sangheili]] forces, fulfilling their mission of removing the Flood from Earth, retreated back to the ''Shadow of Intent'', along with Cortana's message and Guilty Spark.{{Ref/Level|Id=Floodgate|H3|[[Floodgate]]}}
 
With the assistance of 343 Guilty Spark, Cortana's painful message was repaired, revealing a solution to the Flood menace on [[Installation 00|the Ark]]. To Lord Hood's disapproval, Miranda Keyes led UNSC forces allied with Sangheili forces through the [[Portal at Voi]] to the Ark.{{Ref/Reuse|Floodgate}}
 
On the Ark, while the Sangheili and human forces were attempting to infiltrate Forerunner shield control centers and lower the shield surrounding the Ark's Citadel, the infested [[High Charity]] made a [[slipspace]] jump and smashed into the Ark, releasing the Flood onto the superstructure. The [[Gravemind]] then made a brief alliance with the Master Chief and the Arbiter, [[Thel 'Vadamee]], in order to stop the [[Prophet of Truth]] from activating the Halo Array, providing Flood backup to their [[Battle of the Citadel|assault]] on [[Installation 00 Citadel|the Citadel]]. Once Truth was dead, however, the Gravemind treacherously ordered the Flood to turn against the two heroes. The Chief and the Arbiter escaped, however, with some assistance from the Sentinels, who were able to help subdue the Flood with ease. As they escaped the Flood, the Chief and the Arbiter discovered [[Installation 08|a replacement Halo ring]] rising from the [[Foundry (location)|Foundry]] and realized that Cortana's plan to stop the Flood was to fire the new installation at the Ark, an act that would sterilize the local infestation while leaving the Milky Way galaxy unharmed.{{Ref/Level|Id=The Covenant|H3|[[The Covenant (level)|The Covenant]]}}
 
[[File:H3-MC-Boot-FloodBiomass.jpg|thumb|250px|[[John-117]] stepping in Flood biomass as he enters the Flood infested [[High Charity]].]]
In order to fire the new Halo, the Master Chief [[Raid on High Charity|ventured into the derelict and wrecked High Charity]], now a massive Flood hive, where the AI was still held prisoner by the Gravemind. Facing increasing Flood forces and telepathic demands from the Gravemind, the Master Chief was able to rescue Cortana who still possessed [[Activation Index|the Index]] from [[Installation 04]]. In order to buy time to escape and to delay the Flood, the Master Chief detonated High Charity's reactors, destroying the hive and severely injuring the Gravemind while the Chief, Cortana and the Arbiter escaped on a Pelican.{{Ref/Level|Id=Cortana|H3|[[Cortana (level)|Cortana]]}}
 
The Chief took Cortana and the Arbiter to the new Halo ring, joined by [[Sergeant]] [[Avery Johnson]] in the {{UNSCShip|Forward Unto Dawn}} while the surviving humans and Sangheili retreated through the portal to Earth. However, the Gravemind survived High Charity's destruction and began to rebuild itself on the ring while the Flood [[Raid on Installation 08|invaded]], attempting to stop the new ring from being fired. Aided by Sentinels controlled by 343 Guilty Spark, the Chief, Johnson and the Arbiter were able to reach the [[Control Room]]. However, as firing the ring prematurely would destroy it and severely damage the Ark, Spark turned on the three, mortally wounding Johnson before the Chief apparently managed to destroy him. Cortana was able to use the Index from Installation 04 to fire Installation 08 which killed most of the Flood forms on the Ark along with the Gravemind. However, in its final moments, the Gravemind addressed the Master Chief and the Arbiter telepathically, warning them that their victory would only slow—not stop—the Flood. As predicted by Spark, the installation was destroyed by the firing and the Ark suffered heavy damage. The portal collapsed as the Chief and the Arbiter escaped on the ''Forward Unto Dawn'', leaving the Master Chief stranded in deep space.{{Ref/Level|Id=Halo|H3|[[Halo (Halo 3 level)|Halo]]}}
 
===Aftermath===
{{Quote|You've never heard of a major Flood outbreak since the war because of what they do, not because the ugly suckers haven't broken out of their cages since then.|[[Gabriel Thorne]]'s comment on SPARTAN-IV personnel volunteering to join Flood containment teams.<ref name="OSFM">''[[Halo: Official Spartan Field Manual]]'', PART 09: ALIEN FACTIONS, The Flood (p. 172-175)</ref>}}
Even though the Gravemind was destroyed, and all Flood on the Ark apparently eradicated, the Flood still existed on the other Halo rings (excluding Installation 05 which was sterilized by Sangheili forces) and other remaining Forerunner installations. Even though [[Path Kethona]] was sterilized by [[Omega Halo]] near the end of the Flood war, given the Flood's extragalactic origin, it was possible that more pockets of Flood existed outside of the galaxy - or more disturbingly, that other, more distant galaxies may have fallen to the Flood.
 
Though the UNSC kept the Flood's existence concealed from the general population, it did make preparations for the possibility of the Flood's return. The new [[Spartan Operations|Spartan]] branch was designed as a first line of defense against the Flood, with the [[SPARTAN-IV program|SPARTAN-IV]] complement of the {{UNSCShip|Infinity}} running simulations depicting the possible return of the Flood. This is realized through both [[War Games]] [[Flood (gametype)|exercises]] and the interactive [[UNSC Tactical Simulator|TACSIM]].{{Ref/Game|Id=HSA|SA|Detail=[[Halo: Spartan Assault co-op missions|Flood missions]]}} Some Spartans volunteered to join [[Flood containment Fireteam]]s dedicated to going up against Flood outbreaks, with several outbreaks occurring following the end of the war and being successfully contained due to their actions.{{Ref/Reuse|OSFM}} Teams were deployed to quarantine any areas of potential Flood contamination during the UNSC's initial forays onto Halo [[Installation 03]],{{Ref/Book|H4EVG|Page=200}} though it is unclear whether the parasite was actually encountered. The UNSC has otherwise seen the development of the [[Hazops Cyclops]] dedicated to fighting the Flood threat, and the "Flood" variant of the [[Visual Intelligence System, Reconnaissance|VISR]] - given to Spartan personnel involved in Flood containment duties.{{Ref/Game|H5|Detail=Flood visor description}} The UNSC includes a briefing on the Flood in the [[Halo: Official Spartan Field Manual|Spartan Field Manual]], wherein it details protocols such as [[Emergency Contact Protocol Upsilon]] and the [[CORRUPTER]] cryptonym - to be declared in the suspected presence of a Flood outbreak and upon infection of a Spartan, respectively.{{Ref/Reuse|OSFM}}
 
Three days after the [[Battle of Installation 00]], with some Flood spores having survived the [[Raid on Installation 08|firing]] of [[Installation 08]] and the destruction of High Charity's reactor, [[Installation 00|the Ark's]] [[monitor]] [[000 Tragic Solitude]] had the ruined Covenant capital city of [[High Charity]] sealed off with a [[containment shield]] and the Ark's [[Sentinel]]s raze the perimeter and modify the refugia nearby so that no sentient life would survive in the vicinity, thus depriving the Flood of new host bodies were they ever to escape. Tragic Solitude also set a battalion of Sentinels on patrol around the perimeter, not only to keep the Flood contained, but to defend against any future outsiders who might try to enter the wreckage and thus unleash the Flood.{{Ref/Game|HW2|Detail=''Phoenix Logs'' - ''Idle Hands II''}}
 
===Nightmare on the Ark===
{{Quote|I told you NOT to go inside. FOR GOOD REASON!|[[Atriox]] to [[Voridus]]{{Ref/Level|Id=Manifestation|[[Halo Wars 2: Awakening the Nightmare]]|[[Manifestation]]}}}}
[[File:HW2 FloodTankForm Concept.jpg|thumb|250px|A [[Flood tank form]] overwhelms a Warthog.]]
{{Main|Outbreak on Installation 00}}
In early [[2559#June|June 2559]], after the loss of the ''[[Enduring Conviction]]'', the [[Jiralhanae]] brothers [[Pavium]] and [[Voridus]] were tasked to salvage the wrecks around [[High Charity]] for weapons to use in [[Second Ark Conflict|the conflict]] with the humans of the {{UNSCShip|Spirit of Fire}}. The [[Banished]] leader Atriox, aware of the potential danger of the Flood having survived in the city, gave specific orders to only scout the shell and not to go inside lest surviving Flood be disturbed. However, [[Voridus]], like many of the Banished, did not take the threat of the Flood seriously, believing it simply be another lie of the [[San'Shyuum|Prophets]]. Ignoring the warnings of both Atriox and his own brother Pavium, Voridus deactivated the Ark's Sentinel defense network and used a [[Barukaza Workshop Scarab]] to burn a hole in the [[containment shield]] around the former Covenant holy city. When Voridus' troops entered the wreckage to salvage it, they were ambushed by thousands of Flood [[infection form]]s lurking within High Charity which were then set loose upon the Ark. With the Sentinel defense network down, the Flood were left unhindered by the Ark's defense systems.{{Ref/Level|Id=What Could Go Wrong?|[[Halo Wars 2: Awakening the Nightmare]]|[[What Could Go Wrong?]]}}
 
Pavium quickly assembled a force to rescue his brother, defending against overwhelming Flood forces before finally retreating to meet up with Voridus at a Banished drill site near a Forerunner archive that contained a control terminal for the Sentinel defense network.{{Ref/Level|Id=Fighting Retreat|[[Halo Wars 2: Awakening the Nightmare]]|[[Fighting Retreat]]}} As the area was honeycombed with fissures filled with unstable explosive power, Pavium managed to use Banished Salvage Drills to start a chain reaction to burn away the majority of the Flood in the area, allowing Voridus to access the archive.{{Ref/Level|Id=Light the Fuse|[[Halo Wars 2: Awakening the Nightmare]][[Light the Fuse]]}} Inside, despite heavy Flood resistance, Voridus and his troops managed to reactivate the Sentinel defense network which began to cleanse the Flood.{{Ref/Level|Id=The Archive|[[Halo Wars 2: Awakening the Nightmare]]|[[The Archive]]}} However, by this time, the Flood had created a massive [[Proto-Gravemind]] in the area near High Charity that was in danger of becoming a full-fledged [[Gravemind]] while the threat from the Flood forced Atriox to pull back from positions he had established to fight the humans.{{Ref/Reuse|Manifestation}} This Proto-Gravemind retained a measure of the deceased Gravemind's intellect, but the Gravemind's new vessel had to feed and spread in order for the Gravemind to fully manifest once again.{{Ref/Game|Id=IH2|HW2|Detail=''[[Phoenix Logs]]'' - ''Proto-Gravemind''}} Voridus, Pavium and Banished forces, with the help of the Ark's defenses, in particular a [[Retriever Sentinel]], managed to kill the Proto-Gravemind before it could become a Gravemind.{{Ref/Reuse|Manifestation}}
 
With the Proto-Gravemind dead, the Flood threat was greatly diminished. Atriox ordered Pavium and Voridus to clean up their mess and the Banished joined a force of thousands of the Ark's Sentinels in containing the Flood outbreak and sealing off High Charity once more.{{Ref/Reuse|Manifestation}} The Banished and the Sentinels ultimately ended up "scouring" the Flood from High Charity's ruins.{{Ref/Novel|Id=Sacrifice|SoR|Detail=''[[Sacrifice]]''}}
 
==Flood forms==
The Flood is composed of a variety of specialized forms, each filling a different role in combat. The most basic Flood forms are the infection forms, which come in various types and infect hosts by hijacking their bodies and nervous systems.{{Ref/Game|Id=infectors|HW2|Detail=''Phoenix Log'' - ''Pod Infectors''}} Infected hosts then undergo a rapid series of transformations, becoming lethal combat forms. Combat forms are used to subdue and secure additional hosts for infection, and once they have outlived their usefulness as warriors, they transform into carrier forms, spreading the Flood by spreading more infection forms.
 
Once the Flood has amassed enough bodies to create a Gravemind, it is capable of generating [[Flood pure form|"pure" forms]], which are created solely from assimilated nutrients and biomass. The pure forms observed during the outbreak in late [[2552]] were able to spontaneously mutate between three configurations: Stalker, Tank and Ranged forms, although in practice the true extent of their mutability is impossible to effectively catalog.{{Ref/Reuse|wp flood}}
 
The Flood at this point relegate the combat forms to defensive purposes or as additions to the biomass, calcium, and nervous system reserves of the Flood hive, and rely increasingly on the pure forms as their primary instrument of infantry force. However, in order for the pure forms to fully replace the combat forms, the pure Flood population must be large enough and in a state of equilibrium, which is in a constant state of flux depending on Flood gains and losses. Because of this, it is common for the combat forms to serve alongside the pure forms, in addition to the infection and carrier forms, as the acquisition of hosts and technology would likely outstrip the speed in which the Flood can generate pure forms.
 
===Spores===
{{Main|Flood spore}}
Spores are minuscule airborne Flood forms that serve to spread infection and suffocate potential hosts who inhale them.
 
===Infection forms===
Infection forms are the most basic category of the Flood parasite, consisting of several varieties of both ground- and airborne Flood forms.{{Ref/Reuse|infectors}}
 
======Tadpoles======
[[File:H2A Tadpole2.png|thumb|A Flood tadpole in a tank.]]
{{Main|Flood tadpole}}
Pod infectors undergo a tadpole phase of development before growing into full physical maturity.<ref name="onefinaleffort">[http://halo.bungie.net/news/content.aspx?type=topnews&cid=25650 ''Bungie.net'': ''One Final Effort (4/16/2010)'']</ref>
 
======Pod infectors======
{{Main|Pod infector}}
These are among the most basic of Flood forms. They are tasked with seeking hosts, then infect them that mainly on the ground.
 
======Seeder infectors======
[[File:HW2-SeederInfectors.png|thumb|A Seeder infector.]]
{{Main|Seeder infector}}
These are airborne forms of Pod infectors that mainly tasked with infecting airborne creatures.
 
===Pure forms===
These forms are the natural flood form and are produced depending on need and amount of biomass available.
 
======Tank form======
{{Main|Pure form}}
These large tanking forms can only be produced when a Flood outbreak has amassed considerable amounts of biomass and formed a Gravemind. These forms can take and deal massive amounts of damage.
 
======Ranged form======
{{Main|Pure form}}
These flood forms can only be produced once a Flood outbreak has obtained enough biomass to form a Gravemind. These forms attack via bony spikes shot from it's back area, however is immobile.
 
======Stalker form======
{{Main|Pure form}}
These pure forms can only be produced if a Flood outbreak has gathered a large quantity of biomass to creat a Gravemind. These forms are very fast and agile.
 
======Infester Forms======
[[File:Enc22 Infester.png|thumb|An Infester form.]]
{{Main|Flood infester form}}
These forms attack and infect vehicles.
 
===Carrier forms===
These forms of the flood produce various Flood forms.
 
======Carrier form======
{{Main|Carrier form}}
These bloated forms incubate Pod infectors, and release them upon death.
 
======Spawner Forms======
[[File:Enc22_Spawner.png|thumb|A Flood Spawner.]]
{{Main|Flood spawner form}}
These forms can produce a variety of Flood forms on the move.
 
======Flood burster======
[[File:HW2-Burster.png|thumb|A Flood Burster.]]
{{Main|Flood burster}}
These forms can produce a variety of Flood forms and release them once its internal pressure has reached a certain point.


[[Image:Flood carrier.jpg|200px|left]]
===Combat forms===
{{Main|Combat form}}
Once a host of any organic/sentient species has been infected, they can develop into a combat form.


=== Carrier Form ===
===Key minds===
{{Main|Carrier Form}}
{{Main|Key mind}}
When an Infection Form takes over a body that is unsuitable for combat, or once an infected form is damaged beyond use, it becomes a Carrier Form. [[Grunts]], and [[Jackals]] are immediately turned into this form upon infection, due to their weaker physiology they are not deemed viable to become Combat Forms.
Key minds are nodes for the Flood's collective consciousness, ranging from mobile coordinating forms to compound minds spanning entire planets.{{Ref/Reuse|deluge}}
Eventually when a Combat Form has lived long enough, it too will mutate into a Carrier, regardless of damage taken. Carrier Forms perpetuate the Flood species by acting as mobile incubators for new Infection Forms, keeping them safe until they're fully developed. Once the Infection Forms are ready, the carrier simply explodes, sending the new Infection Forms flying into the air. As soon as they are released, they begin searching for new hosts. This is often done close to possible host organisms.


=== Brain Form ===
======Juggernaut======
{{Main|Brain Form}}
{{Main|Flood Juggernaut}}
[[Image:Flood Brain Form.jpg|200px|right]]
The Flood Juggernaut is a large Flood command form directed by a seething mass of infection forms linked to multiple hosts.
One of the rarest types of Flood is the [[Brain Form]]. A Brain Form is unlike all other Flood forms since it lacks any sort of mobility. While the Infection, Combat, and Carrier forms are dangerous and always hostile, the Brain Form is passive to all but the host within its bloated form. When the Flood attempt to control a starship or interface with a computer system, ta Brain Form is created. It normally contains the body of an individual who has knowledge necessary for operation of said vessel. Like the Combat form, it can read the thoughts and memories of its host provided he or she is still alive, and it will sift through every memory until it finds what it needs. If it cannot find the information it requires, it will direct other Flood to bring it a new body to search while it consumes the last body. It interfaces with computers the same way an Infection Form infects a host, through tendrils that attach themselves to the computer's "neural" system.


=== Prophet Form ===
======Abomination======
{{Main|Prophet Form}}
[[File:HW2-Abomination Render.jpg|thumb|An Abomination.]]
[[Image:FloodProphet01.jpg|200px|left]]
{{Main|Abomination}}
Prophet Form is a term used to describe [[Covenant]] [[Prophets]] that have been infected by the Flood. Due to their unusual nature, they are not thought of as Combat Forms in the way infected [[Humans]] and [[Elites]] are (note that the term "Prophet Form" has not been officially used as canon to date). It is also worth noting that both "Prophet forms" seen thus far had only recently been infected, and may have required more time to be assimilated in the conventional way.
The Abomination is a mobile, low-level key mind form capable of combat that coordinates the Flood on a local scale.


It can be concluded based on the concepts illustrated in the [[Halo: The Flood]]and the [[Halo Graphic Novel]] that figures such as Prophets or military [[commander]]s would be spared the common assimilation procedures for the purpose of preserving the individuals knowledge for later use.
======Proto-Gravemind======
{{Main|Proto-Gravemind}}
A biomass of hosts, for the purpose of utilizing their minds to form a collective intelligence. A Proto-Gravemind can grow into a Gravemind given enough time and host bodies.


=== Gravemind ===
======Gravemind======
[[File:HTMCC-H2A AMonumentToAllYourSins.png|thumb|A Gravemind.]]
{{Main|Gravemind}}
{{Main|Gravemind}}
[[Image:Gravemind.jpg|200px|right]]
This ultimate Flood form serves as the coordinating hive mind of the Flood. While they vary in size and complexity, at their height Graveminds can engulf entire worlds.
At present it is not known what the Gravemind's purpose truly is. It is speculated that the Gravemind is comprised of a huge mass of flesh and bio-matter that Flood combat forms have to one place for absorption. It has a "mouth" through which it communicates, and has many far-reaching tentacle appendages.
 
==Biology==
[[File:Flood wall.jpg|thumb||150px|A wall covered in heavy Flood biomass: evidence of advanced infestation.]]
[[File:H2A Render FloodSpore.png|thumb|150px|A Flood spore.]]
 
The Flood is an anomaly in all known biology; their mode of growth and reproduction would make them incompatible with any natural ecosystem.{{Ref/Reuse|bestiarum}} They are extremely adaptive, and transform hosts to suit the species' current needs.
 
Capable of surviving extreme environments ranging from -75 to +53 degrees Celsius and even underwater, the Flood can withstand the varied environments of all known inhabited planets. However, the Flood seem to thrive the most in moist and humid areas; it is thought that the extremely cold artificial weather patterns near certain important areas of the Halo rings are intended to hinder the Flood. {{citation needed}} In order to give themselves ideal living conditions, the Flood will transform the environment to better suit their needs, altering the atmosphere and covering solid surfaces with Flood biomass.
 
All Flood biomatter is made of a unique, undifferentiated [[Flood Super Cell|"Super Cell"]], which relays information and coordinates the cellular assembly like conventional neural cells. It is also capable of flexing and moving in coordination with other cells, and can be arranged to mimic any bodily organ that is required for the Flood's use. This combination of traits has caused some to refer to it as "thinking muscle."
 
The Flood Super Cell physically resembles both neuron and glial cells in structure, with a central cell body and many tendril-like, branching structures.
 
===Infection===
[[File:Flood Cell Division.jpg|thumb|left|150px|[[LF.Xx.3273 Flood Super Cell|Flood Super Cells]] performing [[Wikipedia:Cell division|cell division]] inside of a host.]]
[[File:Infection Form.jpg||thumb|left|150px|The [[Pod infector]]; the primary method administered towards the infection of an organism.]]
In order to reproduce and grow in biomass, the Flood needs to infect and assimilate other life-forms. While capable of infecting nearly any organism, the Flood shows a strong preference for [[sentience|sentient]] life-forms, as assimilating their cerebral tissue allows the Flood to increase its overall intelligence.
 
The first stage of a Flood outbreak is typically initiated by contact with Flood spores or even more reduced forms of Flood biomatter, should no extant Flood forms be present.<ref name="wp flood">[https://www.halowaypoint.com/en-us/universe/species/flood ''Halo Waypoint'': ''Flood'']</ref> Once a basic infestation has been established, Flood infection is typically carried out by [[Pod infector]]s, which are some of the most complex forms that can be generated from Flood biomass during the [[Feral Stage]]. Upon sighting a host, a Pod infector will leap for the most accessible area of the victim's torso; if the target is an armed human or other bipedal species, this is typically the left side of the chest below the neck, as most combatants will expose this area while shouldering their weapon.{{Ref/Book|AoBW}} Upon contact, the Pod infector will rasp away at clothing and flesh until it has burrowed into the chest cavity, mortally wounding them. As the host dies, the Pod infector extends tentacles that tap into the victim's spinal cord or other nerve center, forcing a "match" with the nerve signals previously produced by the host's living brain.
 
To begin the physiological conversion of a host, the Pod infector injects encapsulated Flood Super Cells into the body. As it does so, the form analyzes the entire genetic code of its host, allowing the Flood to determine the most effective use of the victim. Simultaneously, the inserted Flood cells capture and break down the host body's cells into organic raw material, which is then absorbed and assimilated into the attacking Flood cells.
 
The Flood infection then uses the absorbed biomass to create new masses of Flood Super Cells. As the Flood continues to use the resources of the host body, particularly its calcium reserves, the parasite's own cells build upon and augment the framework of the host to produce one of a variety of Feral-Stage Flood forms. This entire process, from the initial kill to total control over a fully mutated host body, takes only a matter of seconds.{{Ref/Book|AoH|Page=28}}{{Ref/Book|AoH|Page=50}}
 
The infection process is not limited to living hosts; organisms that are already deceased are also susceptible to infection. As long as the host has not decomposed to the point where the body and nervous system have completely deteriorated, the Pod infector can infect and convert the host.
 
The Flood infection utilizes the host's biological content as the fuel for its work, which is consumed and employed at prodigious speeds. From the beginning until the end of this complex, multi-step process, the Pod infector keeps the body "alive" by chemically isolating the host's dying brain, preventing the signals of brain death from reaching the rest of the body and causing it to shut down, which is an essential part of the parasite's mission of consuming and converting the host organism.{{Ref/Book|AoH|Page=48}}
 
While the Pod infectors are an effective and economical means of delivering infection, particularly against armored targets, the Flood can infect hosts through other means. [[Flood spore]]s, if inhaled, can transform a host into a combat form. This is the Flood's primary means of infection when an outbreak reaches a certain point: Flood spores will be deployed into a local biosphere in copious quantities, instantly overwhelming any unprotected biological targets.{{Ref/Novel|Id=s231|Sil|Page=231-232}} Flood conversion may also be initiated through contact with lacerated tissue, making any Flood form a potential vector for infection.{{Ref/Reuse|mona lisa}} Even the most basic form of Flood genetic material is virulent; once inserted into a host, it will mutate the host's DNA over the course of generations until it is capable of forming its own Flood super cells.{{Ref/Novel|Cry|Page=268-270}} When infected by these mechanisms, host organisms tend to mutate at a more restrained level that maintains the structure and form of the infected life form, albeit with a significant number of haphazardly placed tendrils, claws, and minor organs, such as sensory stalks.{{Ref/Comic|HGN|Page=19-54}}
 
===Neurological===
Pod infectors selectively target species that possess sentient intelligence and are of sufficient biomass, and can infest intact or lightly wounded dead bodies. As a host organism is killed by the initial attack, Pod infectors use tendrils that pierce the skin of the host and find their way to the spinal cord. They then synchronize with the host's nervous system and gain control of their body, replacing its now-absent consciousness with the ravenous and voracious psyche of the Flood. Any useful information present within the memory centers of the host's brain, such as battle strategies and technical knowledge, is retained for use by the Flood. Despite this lingering of certain memories, no trace of the original mind remains; only a simplistic and primal urge to assimilate other species drives the organism after infection. However, the entirety of the host's mental content is seemingly absorbed and added to the whole, as the Gravemind has shown to be able to simulate its victims' consciousnesses within itself.{{Ref/Reuse|Gravemind}}{{Ref/Novel|Sil|Page=251}}
 
In isolated cases, such as when the Flood seek very specific information from an individual's mind, they utilize an alternate method of infestation that does not subdue the host's consciousness immediately despite mutating their physical form; this allows the Flood to "burrow" into the host's mind, eventually giving them access to the entire scope of the host's memories. This has only been specifically seen when the Flood sought information from Captain [[Jacob Keyes]], such as the location of Earth, or a starship that would free them from [[Installation 04]].{{Ref/Novel|TF|Page=175}} In addition, if the Pod infector that initiates assimilation is extremely aged or damaged somehow, the host may remain alive and conscious; Private [[Wallace Jenkins]] suffered such a fate, remaining alive and fully aware of his situation despite the mutation of his form, even exhibiting limited control over his body at times when the Flood instincts were dormant.
 
When the Flood have amassed enough sentient hosts to form a Gravemind, then their behavior becomes far more coordinated, and the Flood collective becomes much more deadly as a result. The Gravemind may also speak directly through combat forms and [[pure form]]s under its control.
 
===Physiological===
While baseline neurological assimilation is the same regardless of host species, the Flood infection process includes physiological transformations, which vary depending on the host and the Flood's present need for specialized combatants.
 
Upon being infected, higher-level species capable of combat are typically transformed into combat forms. The host's physiology is completely "rewritten" during the infection process: organ-based systems are corrupted, organ-specific functions are decentralized, and body cavities are decayed, making hitbox-selective incapacitation impossible and brute force incapacitation difficult. Due to the lack of a central nervous system, decapitation will neither stop nor slow a combat form.
 
Furthermore, munitions with high speed and penetration, such as sniper rifle rounds, are usually ineffective against combat forms. Picking off selective areas of a combat form leads to no wide-scale physiological impediments, and the projectile will punch through the decayed flesh rapidly, exiting through the other side of the combat form without dealing any major damage.
 
To facilitate melee combat, combat forms generate tentacles and claw-like structures, which protrude from the host's arms. Generally, a single arm develops into a clawed appendage, while the other retains the capacity to manipulate objects, such as firearms. The rapid growth of these structures tends to push aside and break apart the host's hands and forearms.
 
Combat forms can strike with devastating force, move with tremendous speed and agility, and jump to great heights, displaying physical abilities far beyond those of the host organism while it was alive.
 
The price for these formidable abilities is that the required metabolism is so rapid and extreme that the host organism's corpse is rapidly broken down by the parasite. This is dramatically demonstrated by the rapid decomposition and disintegration of an infected life-form, which will eventually become a stunted, bloated, mobile generator and incubator for more Pod infectors, which is commonly known as a [[Flood carrier form|carrier form]].
 
Pod infectors and pure forms sense their surroundings using specialized tentacles, which are tipped with red ganglia. Exactly how these organs work is unknown; they may provide visual or chemical information, or perhaps a combination of both. In combat forms, these tentacles typically protrude from the chest cavity where the Pod infector is housed; in pure forms, they are located on the front of the "head" segment.
 
The described changes occur when the Flood has still yet to overwhelm any meaningful resistance in a given locale. When this has been accomplished, available organisms will no longer be used to create purpose-built combat forms but are instead used to create a Gravemind or simply broken down and processed into biomass for a [[Flood hive]].{{Ref/Reuse|mona lisa}}{{Ref/Reuse|s231}}
 
===Metaphysical and technological===
{{Quote|The Flood changes everything. Not just flesh. Space itself is infected. That's the power the Precursors once had...  isn't it?|The [[Ur-Didact]] to the [[IsoDidact]].{{Ref/Novel|Id=s223|Sil|Page=223-224}}}}
Although the Flood is biological on the most basic level, as an outbreak advances, it becomes a highly powerful self-replicating swarm which will seek to harness any mechanism — philosophical, ideological or technological — to perpetuate itself and its goals. These aspects were most prominently demonstrated during the two major outbreaks in the Forerunner era, particularly the [[Forerunner-Flood war]].{{Ref/Novel|Id=s214|Sil|Page=214-215}}
 
The Flood's most notable meta-biological aspect is the [[logic plague]], an abstract infection carried along information transmission and exchange, typically used to subvert AIs. Mechanisms used to perpetuate this infection range from subtle philosophical persuasion through mutual conversation to more predatory techniques, such as swarming AI networks with a self-replicating information infestation with the ability to convert most machine intelligences near-instantaneously. In the later days of the Forerunner-Flood war, the Flood was able to spread the logic plague through any of its biological forms or infected [[ancilla]]s.{{Ref/Reuse|s214}}
 
During its war with the Forerunners, after accumulating sufficient intellectual capacity and processing power in the form of several planetary [[Key Mind]]s, the Flood gained the ability to manipulate [[neural physics]], which served as the foundation of [[Precursor]] technology.{{Ref/Reuse|key mind}} This enabled it to control the Precursors' formerly dormant structures—most prominently [[star road]]s—and use them as weapons against the Forerunners, as well as manipulate space-time to become incompatible with Forerunner [[Slipstream space|slipspace]] travel. On a more abstract level, the Flood's connection to neural physics caused the Forerunners to experience the very space-time around them as being increasingly hostile.{{Ref/Reuse|s223}} Upon the development of a Gravemind, the Flood becomes capable of what can be described as telepathy among its hosts;{{Ref/Reuse|bestiarum}} given the Flood's origins, this ability may be related to neural physics.
 
The Flood is able to use any technology it has assimilated, often to a superior degree than the original creators of said technology due to its vast base of knowledge and intellect, as evidenced by its ability to transition the [[UNSC frigate]] ''[[UNSC In Amber Clad|In Amber Clad]]'' into ''[[High Charity]]'' with a precision slipspace jump normally unheard of with human ships. Similarly, it was capable of tapping into [[Installation 05]]'s [[teleportation grid]] and utilizing it with great accuracy.{{Ref/Reuse|Gravemind}}
 
===Developmental stages===
A Flood infestation goes through four distinct developmental stages:
 
#During the [[Feral Stage|'''Feral''']] stage, the Flood are at their simplest form: they communicate via pheromones and have the instinct to harvest enough [[calcium]] to establish a viable Gravemind.{{Ref/Reuse|bestiarum}}
#The Flood become truly dangerous during the [[Coordinated Stage|'''Coordinated''']] stage: at this point, they are controlled by the Gravemind that was created in the first stage.{{Ref/Reuse|bestiarum}}
#In the [[Interstellar Stage|'''Interstellar''']] stage, the Flood take control of space-faring technology which they use to consume the local star system. Subsequently, they spread throughout the galaxy to infect more hosts.{{Ref/Reuse|bestiarum}}
#The [[Transgalactic Stage|'''Transgalactic''']] stage is a theoretical stage in which the Flood utilizes all captured technology to depart to uninfected galaxies to further replicate.{{Ref/Reuse|bestiarum}}
 
{{Flood}}
 
===Countermeasures===
[[File:Flood Eat.jpg||thumb|250px|A Marine being attacked by a Pod infector.]]
====Small-scale====
There is no confirmed way to effectively stop a Flood infection once Flood Super Cells have entered a host. The only exception is too rare and factually suspect to be considered viable: the human disease "[[Boren's Syndrome]]", a neurological condition where radiological exposure causes electrical anomalies in the host nervous system, allegedly renders the host immune to infection while living. The neural frequencies of the host are "scrambled", apparently without negative consequences to the host. However, during Flood infection, the Pod infector is unable to tap into the host's nervous system, rendering it incapable of proceeding to large-scale physiological mutation. An interesting side effect of this failed infection is that Flood DNA and cells appear to be harmlessly integrated into the host's body. This seems to provide the host with increased regenerative capabilities, although this has not been independently verified. While Boren's syndrome left untreated may render the victim immune to the Flood, leaving it untreated almost always equates to death within a couple of years. There is said to be a "one in a billion" chance that research into the disease could produce a cure.{{Ref/Novel|FS|Page=244-246 (2001 edition)}} However, the accuracy of this information remains highly dubious, as most of it is simply a hypothesis devised by [[Catherine Halsey|Dr. Halsey]] to explain how [[Avery Johnson|Avery J. Johnson]] survived Flood infection. Johnson's "Boren's Syndrome" was actually a story invented to cover his past in the [[ORION Project]]: his symptoms were side-effects of his experimental enhancements, not a neurological disease.{{Ref/Comic|HGN|Detail=[[Halo Graphic Novel, Page 122|page 122]]}}
 
While the Forerunners could not find a cure for Flood infection, the [[Composer]] was designed to eliminate its effects by [[Mind transfer|extracting]] a Flood victim's consciousness into digital form and then re-inserting it in an artificially-constructed body devoid of infection. However, the Forerunners never perfected this process and any imprinted bodies died soon after,{{Ref/Novel|Sil|Page=40}} due to the Flood somehow resuming the infection in the new body. The Forerunners attempted to use various means to slow down this process, including a specially designed harness of mesh and wires, which would apparently counteract the infection. The victim would retain most or all of their mental faculties and self-control, although their body would continue to be mutated by the Flood. This countermeasure was only temporary, however, and the victim would eventually either decay and die or be fully subsumed by the Flood. Several Forerunners on Installation 07 were subjected to this countermeasure.{{Ref/Novel|Pri|Page=268}}{{Ref/Novel|Pri|Page=322-323}}
 
Although initially used to treat those with small-scale Flood infection already present, the Composer was eventually used by the [[Ur-Didact]] to convert his [[Promethean]]s, as well as a large number of humans, into [[artificial intelligence|AI constructs]] for his [[Promethean Knight|war machines]], in an effort to combat the Flood more effectively and without risk of infection.{{Ref/Reuse|terminals4}}


It is theorized that the Gravemind may in fact control other Flood forms, the 'hive mind' of the Flood, or at least influences each form via telepathy. It is also speculated that the Gravemind may be a highly evolved Flood Brain form.
====Large-scale====
Since the Flood are a parasitic life form, they depend on other organisms for survival. Thus, the most effective way to stop a large-scale infestation is to take away their food sources. The Forerunners first attempted this by bombing large population centers and destroying inhabited star systems via supernovae, but this was seen as one of their last resorts and as such was performed too late in the war to make much difference. The last resort was the use of the Halo Array, which destroyed all sentient life in the galaxy and starved the Flood into extinction. However, the Forerunners' plan included some measures to preserve sentient life and return them to their home planets, allowing the Flood's "food source" to persist. Unfortunately, at the same time, specimens of Flood spore cells were kept for analysis on the Halo installations, allowing the threat of the Flood to be merely forestalled and not eradicated.


The Gravemind was discovered on [[Delta Halo]] in October of 2552. It "saved" [[Spartan-117]] after he fell into the lake under which the Gravemind resides. It also "rescued" the [[Arbiter]] as he fell to his doom within the depths of the [[Library]]. It forced the two of them to cooperate in order to stop the Prophets of [[Prophet of Mercy|Mercy]] and [[Prophet of Truth|Truth]] from activating [[Delta Halo]]'s primary weapon.
Environmental stimuli seem to have an effect on Flood metabolism. In the event of a Flood outbreak, Halo installations create a [[Quarantine zone (location)|quarantine zone]] around the affected area,{{Ref/Level|H2|[[Quarantine Zone]]}} inside which the ambient temperature is drastically lowered. [[Cold Storage|At least one]] [[Flood research facility]] used low temperatures to keep its specimens inactive and dormant as it studied them.{{Ref/Game|H3|Detail=multiplayer level ''[[Cold Storage]]''}} At least [[Control Room (Installation 04)|one]] of the Halo installations' [[Control Room]]s was located in a snowy environment, possibly to hinder escaped Flood in their attempts to reach it.


[[Cortana]] later deduced that the Gravemind had in fact duped everyone. It needed a method of escaping Intallation 05 and while the Master Chief, The Arbiter, Sgt. Maj. Johnson and [[Commander Keyes]] were trying to stop Truth and Mercy it had the Flood take over ''In Amber Clad'', Keyes' ship. It used the ship to move itself from Delta Halo to [[High Charity]], the Covenant holy city.
The Flood are also vulnerable to temperatures on the other end of the spectrum. [[plasma weaponry|plasma weapons]] will easily burn through Flood tissue; the Forerunners incorporated directed-energy weapons into their [[Sentinel]]s, indicating that they had determined high-enthalpy energy weapons to be the best 'basic countermeasure' available to fight the Flood. Known examples of Forerunner personal weaponry appear to be geared toward the disintegration of organic matter,{{Ref/Game|H4}} also reflecting the Flood's vulnerability to extremely high temperatures. The UNSC has taken advantage of this weakness as well, making heavy use of [[flamethrowers]] in their engagements with the Flood.


The Gravemind is either made of countless bodies that were unsuitable as Combat or Carrier forms or maybe the last Forerunner Flood form in existence. [[The Art of Halo]]: Creating a Virtual World states that the Gravemind is “literally built from the bodies of its enemies and its own fallen warriors reassembled into a massive, tentacled, and intelligent entity."
When engaging heavily entrenched Flood structures such as [[Flood colony|colonies]] and [[Cocooned base]]s, orbital bombardment is usually a preferred method of elimination as direct attacks can be extremely costly.{{Ref/Game|HW2|Detail=''[[Phoenix Logs]]'' - ''Cocooned Base''}}


It is not known how that Gravemind managed to survive the first Halo activation, as he(or it) being a sentient life form should have died with the rest of the Forerunners as well.
====The "cure"====
During the [[human-Forerunner wars]], humanity (who at the time was a [[Prehistoric human civilization|technologically advanced, spacefaring race]]) believed they had discovered another way to defeat the Flood. Using genetic engineering, a third of the human population was altered to carry a set of specially designed genes. This group of humans was then allowed to be infected by the Flood. Once the genes were exposed to Flood DNA, they triggered a cascade of cell destruction that spread throughout all Flood biomatter. The majority of the Flood died off as a result, although they would return in full force ten thousand years later. All information regarding this "cure" was destroyed by the humans as a final act of revenge against the Forerunners, leaving them unprepared for the second Flood strike.{{Ref/Novel|Id=cry267|Cry|Page=267-272}}


=== Juggernaut Form ===
In reality, while humanity's efforts indeed produced the intended result and repulsed the Flood threat, it was later revealed by the [[Primordial]] that the Flood retreated by its own initiative, as humanity was not yet to be "tested" by the Flood. As such, humanity's survival was in fact due to the Flood's deliberate decision not to infect human populations, as the Precursors had determined to use the Flood to punish the Forerunners first.{{Ref/Reuse|p364}}
{{Main|Flood Juggernaut}}
 
[[Image:Juggernaut fight.png|200px|left]]
After the Flood began to ravage the Forerunners' ecumene, Forerunner scientists led by [[Master Builder]] [[Faber]] tried in vain to extract information on the cure from humans' ancestral memories, imprinted as part of their ''[[geas]]'' by the [[Librarian]], unaware that the cure never really existed.
The Flood Juggernaut is a large Flood form found encoded in the Halo 2 disc (the same way as the Engineers in Halo 1). The Juggernaut Flood is huge, at least twice as tall as the Master Chief.
{{Clear}}


== History ==
==Combat behavior==
[[File:Floodambush.jpg|thumb|250px|The Flood emphasizes numerical superiority in engagements.]]


Next to nothing is known about their origins. Some think that they are some kind of primitive life form accidentally discovered by the ancient [[Forerunner]] race. Others believe that they were a [[Forerunner]] attempt at cloning or resurrection. However, their potential for destruction is not in doubt. Their all-consuming nature and their dependance on sentiant life means that it would be impossible for the Flood to be a part of any conventional ecosystem. As such, it is not unreasonable to assume that the Flood is not entirely natural if at all.
The Flood's combat strategy is simple: they throw themselves at potential hosts in huge numbers and with any and all weapons available. The parasite seeks to saturate its opponents with an excess of noise, so thoroughly it impedes the enemy's ability to process information and prevents them from organizing a meaningful resistance.{{Ref/Reuse|terminals}} The Flood exhibits this behavior on all levels, from its physical swarm tactics in both ground and space to its corruption of data networks and, once an outbreak has progressed far enough, space-time itself.{{Ref/Reuse|s223}}


Flood tactics are based primarily on asymmetrical warfare, using various different forms to kill and infect enemy forces. Infection forms swarm across the battlefield in waves, reanimating any corpses and victims they can find, while combat forms attack and kill any living hosts in sight. Pure forms, if present, provide support to combat forms in a number of ways: Stalker forms move to strategic positions and mutate into either Tank or Ranged forms, which can destroy enemy vehicles or fortifications or provide suppressing fire from afar.


This much is known; the Flood were kept by the Forerunners for scientific study on the [[Halo]]s as well as other structures. The Halos are an immense weapons array designed for the sole purpose of wiping out the Flood. Although the intention was such the fact that the Flood still exists indicates that the Forerunner's plan may have failed.
The Flood have a strong preference for close-range combat: combat forms possess extremely powerful melee capabilities due to their specialized claws and tentacles, and are capable of bringing a suit of MJOLNIR armor's energy shields to half strength with a single strike. Exploding carrier forms cause tremendous damage at close range, with the added benefit of spewing Pod infector in the immediate vicinity of enemies. Pod infectors are capable of attacking only at point-blank ranges, exploding in damaging bursts when meeting an opponent's personal [[Energy shielding|energy shields]] or instantly grabbing hold of an unshielded enemy organism and infecting it.  


== Assimilation ==
[[File:Catch.jpg|thumb|250px|left|Typical Flood combat behavior: ravenously targeting the nearest enemy, with Pod infectors running ahead of combat and carrier forms.]]
[[Image:Infectionforms Halo1.jpg|thumb|Infection Forms consuming corpses]]
Combat forms can also use firearms, and have been seen clutching them in one of their hands (regardless of whether it is one-handed or two-handed). However, combat forms are very inaccurate when using ranged weaponry; it is believed that the neurological and physical effects of Flood infection significantly decrease accuracy.  
The Parasite takes over the bodies of hosts and assimilates them into the  Flood. This is done via Flood Infection Forms. Infection Forms have long tendrils that pierce the skin of the host and find their way to the spinal cord. From here they synchronize with the host, taking over brain functions to prevent resistance of any kind. After this time, the original mind is erased memory by memory, until nothing is left but a "hungry" Flood form. Separate tendrils release aggressive spores laced with Flood DNA into the host, causing the body to mutate into a Combat Form or a Carrier Form.  


Combat Forms are the closest thing to what we would deem as a Flood "military". It's sole purpose to attack and later capture more able host bodies. Of the many species in Halo, only two are known to produce fully matured Combat Forms; the [[UNSC|Humans]] and the [[Elites]]. The [[Grunts]] are believed to be mutated into Carrier Forms because they are too small and weak to be effective as Combat Forms. While there has been no evidence as to why the Flood have not shown assimilation of other species, many theories abound. [[Jackals]] are believed to have a too porous, bird-like bone structure. [[Brutes]] are believed to lack sufficient neural function. [[Drones]] would provide more than sufficient calcium stores in their insectoid exoskeleton, though it would probably be nearly impossible for flood infection forms to penetrate, even if they could actually catch these quick flyers. Some believe that the Flood have certain "tastes", similar to that of Humans. [[Hunters]] cannot be successfully assimilated by the Flood due to their bodily makeup consisting of hundreds of invertebrate entities. It is speculated that other, larger races, such as [[Drinol]] or [[Sharquoi]] become larger Flood organisms, such as [[Flood Juggernaut]]s.
The Flood collectively learn any information contained within the minds of their sentient hosts. For example, during the ill-fated raid on the ''[[Infinite Succor]]'', after even one Sangheili was assimilated, all the Flood could open any doors and access any system on the ship immediately afterward, demonstrating the Flood's ability to share collective knowledge across their species. Furthermore, when the Flood assimilated the Prophet legate, the [[Minister of Etiology]], the Flood reflected the high-level knowledge of the Legate through its individual organisms to Special Operations Commander Rtas 'Vadumee, mocking him with the words, "''the Forerunner could not defeat us...what chance you?''". The Flood, however, reflect unusual knowledge in the individual psychologies of its individual species in processes facilitating increased infestation: The Flood were sighted gathering bodies in the corners of the Covenant ship ''Truth and Reconciliation'', perhaps making it easier for infection forms to find hosts to reanimate. The Flood were also gathering body parts for a new proto-Gravemind aboard the ''Infinite Succor''.


=== The Effects of Assimilation on the Host's Body ===
Flood combat forms have been sighted crudely operating {{Pattern|Karo'etba|Ghost}}s, [[M12 Warthog|Warthog]]s, [[Zurdo-pattern Wraith|Wraiths]], and [[Scorpion]]s in ground operations, although with slow deliberation and clumsy driving and accuracy. The assimilated crewmen of the [[UNSC frigate]] ''[[UNSC In Amber Clad|In Amber Clad]]'' were also able to initiate a [[Slipspace]] micro-jump for the frigate and perform rudimentary maneuvering of the ''In Amber Clad'', navigating the frigate from the atmosphere of [[Installation 05|Delta Halo]] to ''High Charity'' and then crashing the vessel into a tower, releasing Pelican dropships to rain down upon the Covenant capital.


The book [[Halo: The Flood]] describes the effects of assimilation on the human body in great detail.
While one of the Flood's main strengths lies in the decentralization of its compound mind and ability to be reborn from even the most carefully reduced remnant, it is nonetheless known to develop and assume a certain "core" Gravemind as an embodiment for its strategic intelligence. The Forerunners observed that the Flood exhibits a seemingly instinctive tendency to recoil and concentrate all of its otherwise distributed assets around this core when threatened on a large scale. A threat to the Flood's existence major enough to prompt such action was the imminent firing of the Halos, at which point the Flood gathered all of its available assets and launched an attack on the Ark.{{Ref/Game|H3|Detail=[[Terminal (Halo 3)|Terminal 3]]}}
*First the host's extremities grow numb and their lungs feel heavy. Then the greater Flood intelligence begins to take over the mind<ref>[[Halo: The Flood]], page 175</ref>.
*The host's vision seems to be "a gauzy white haze", but the host can make out shapes<ref>[[Halo: The Flood]], page 197</ref>. The host can hear a low droning through the ordeal.
*The limbs feel to the host as if they are made of a spongy liquid. Their lungs itch as well<ref>[[Halo: The Flood]], page 198</ref>.
*The Flood controlled [[Combat Form]] can generate "tentacles" at a rather rapid pace, in total disregard for the host's skeletal structure<ref>[[Halo: The Flood]] page 222</ref>.
*There also seems to be an agent released into the host's blood stream that softens bone tissue.<ref>[[Halo: The Flood]] page 223</ref>
*Combat forms also seem to resemble their host before the infection to a degree.<ref>[[Halo: The Flood]] page 224</ref> 


The only known way to effectively stop a Flood infection is to have [[Boren's Syndrome]], the radiation-caused disease that [[Sergeant Major Avery J. Johnson|Sgt. Johnson]] has. Boren's Syndrome does not completely deny the infection, however, which is proven by the fact that Johnson has an increased, but still limited, regeneration ability. However there appears to be some debate as to the validity of this condition. There is some evidence to support the accusation that Boren's Syndrome was in fact a cover for Sgt. Johnson's speculated status as a [[Spartan 1.0]].
The Flood is very effective on a psychological level, as its parasitic nature and gruesome appearance alone are a source of horror for most of its enemies and thus serve as powerful demoralizers. It is not uncommon for survivors of Flood encounters to be severely traumatized, often to the point of delirium, after witnessing their comrades get infected or being subsequently forced to kill said infected individuals.{{Ref/Level|CE|[[343 Guilty Spark (level)|343 Guilty Spark]]|Detail=[[Paranoid Marine]]}}{{Ref/Level|H3|[[Floodgate]]|Detail=[[Suicidal Marine]]}}
{{Clear}}


== Species Behavior ==
==Notable victims==
{{col-begin}}
{{col-2}}
*[[Wallace Jenkins|Private Wallace A. Jenkins]]
*[[Riley|Private Riley]]
*[[Jacob Keyes|Captain Jacob Keyes]]
*[[Manuel Mendoza|Private Manuel Mendoza]]
*[[Eugene Yate|Lance Corporal Eugene Yate]]
*[[Cranker]]
*[[Gersten]]
*[[Orlav]]
*[[Prophet of Mercy]]
{{col-2}}
*[[Prophet of Regret]]
*[[Prophet of Truth]]
*[[Minister of Etiology]]
*[[Bero 'Kusovai|Sub-Commander Bero 'Kusovai]]
*[[Bok]]
*[[Mendicant Bias]]
*[[2401 Penitent Tangent]]
*At least one Spartan from [[Fireteam Leviathan]]
{{col-end}}


=== [[Halo: Combat Evolved]] and [[Halo 2]] ===
==Non-canon and dubious canon appearances==
[[Image:Catch.jpg|250px|thumb||left|Typical Flood attack strategy; swarms of Combat Forms in front with Infection forms following]]
===Silver Timeline===
Though the Flood can survive in environments that are suitable to both [[Covenant]] and [[Humans]], they prefer to alter it to their own preferences. This happens when the Flood release deadly spores that only they can breathe. Only someone wearing a hermetically sealed suit or other protective garment can survive through the change. The Flood can alter the atmosphere of a planet, transforming it so non-Flood life forms can no longer survive, thus they become easy prey for the Flood. After the Flood was released on Halo they began to change it, but thankfully they did not succeed.
{{Notice/Silver}}
{{Quote|no=two|q1=What is it?|Kwan Ha|q2=It is the cohesion. The meaning. The final equilibrium. This is the end. This is the Flood.|The Mother|The Mother reveals the monster's true identity.{{Ref/Reuse|TVHalo}}}}
[[The Mother]] repeatedly warns [[Kwan Ha]] of a "monster" that is approaching in visions.{{Ref/Film|TV|Episode=[[Aleria (TV Series)|Aleria]]}}{{Ref/Film|TV|Episode=[[Onyx (TV Series)|Onyx]]}}


The Flood is a very intelligent race, gaining the knowledge of each host they infect. In addition, they are under the control of the Flood "puppet master" The [[Gravemind]] (this is revealed in the Halo 3 plot summary on bungie.net).
After accessing a Forerunner site on [[Onyx/Silver|Onyx]], Kwan, Dr. [[Catherine Halsey/Silver|Halsey]], and [[Miranda Keyes/Silver|Miranda Keyes]] are shown a [[star map]] documenting the Flood's spread, causing Kwan to realize that the monster is real.{{Ref/Film|TV|Episode=[[Thermopylae (TV Series)|Thermopylae]]}}


=Battlefield Tactics=
An overeager biologist named [[Janine Caryyew]] inadvertently releases Flood spores from within a sample container collected by Kwan, Halsey and Miranda, unleashing the Flood throughout the Onyx base. The Flood quickly consumes and converts most of the base's personnel, including [[Admiral]] [[Margaret Parangosky/Silver|Margaret Parangosky]]. While Halsey is infected, Miranda places her mother into stasis in the hopes of finding a cure. Kwan, [[Soren-066/Silver|Soren]], [[Kessler]] and [[James Ackerson/Silver|Ackerson]] manage to escape thanks to the intervention of the Mother who warns Kwan of the true nature of the Flood, but [[Laera]] remains behind after getting infected.{{Ref/Film|Id=TVHalo|TV|Episode=[[Halo (TV Series)|Halo]]}}


*The [[UNSC]] tends to use firepower rather than numbers, preferring a tactic of using a few strong, well trained units with a lot of firepower to achieve their goals.
==Gameplay==
{{Main|The Flood/Gameplay}}


*The [[Covenant]] tend to outnumber and overrun their enemies, relying more on the bravery and courage of their loyal soldiers rather than pure firepower.  
===Changes from Halo: ''Combat Evolved'' to ''Halo 2''===
*In addition to making low growls, the Flood make screeches and roars, which were not present in the first game.
*Their skin is more textured due to the improved graphics engine, giving the Flood a more realistic and necrotic look. The faces of the human forms are also more readily recognizable.
*If a combat form has both arms shot off, it will walk slowly then explode, releasing the Pod infector that was controlling it. This eliminated the Flood Buddy glitch.
*Rather than having to "gather bodies," the Flood can seemingly infect hosts over the course of a few minutes or even a few seconds, once again possibly due to the presence of the Gravemind, eliminating the need to gather bodies to form one.
*Rather than corpses reanimating themselves, a Pod infector will enter the body, then reanimate it, even if the form was just eliminated.
*If the player is attacked by a Pod infector while their shields are down, the character will die instantly. In ''Halo: Combat Evolved'', it would simply take a bar of health away.
*Some combat forms derived from Elites still have functional energy shields. However, they never possess active camouflage, as some in ''Halo: Combat Evolved's'' [[The Maw |final level]] did.
*Flood combat forms can use vehicles, albeit clumsily, and are also able to board the player's vehicle; they will then proceed to melee the player character and pull them from the vehicle.
*Melee attacks do little to no damage against the Flood in ''[[Halo: Combat Evolved]]''. They deal a bit more damage in ''[[Halo 2]]''.
*The Flood prefer to stay back and fire on the player rather than rush him. This could be because of the presence of the Gravemind, though, from a gameplay perspective, it is due to improved artificial intelligence.
*In ''Halo: Combat Evolved'' a combat form would walk slowly when shooting at the player. When losing the arm that shoots, it would charge the player character. In ''Halo 2'', combat forms either use a melee attack or shoot at the player in these situations.
*If a player is wielding an energy sword or shotgun, a combat form may retreat. They will not do this if a player is wielding other weapons.
*Unlike in ''Halo: Combat Evolved'', shooting one Pod infector will not result in the rest of the swarm exploding with it. However, Pod infectors are encountered in proportionally smaller numbers in comparison to ''Halo: Combat Evolved''.
*Pod infectors appear noticeably less textured and detailed than in ''Halo: Combat Evolved''.
*Flood Combat Forms do not "feign death" anymore.
*Splattering become more difficult as the Flood can use the vehicle momentum to hijack the vehicle.


*The Flood on the other hand have no obvious strategy. They fight without fear of death or injuries (a lone Combat Form will sometimes sacrifice itself to advance the cause of the battle). They attack by finding a group of enemies and then send everything they have at them hoping for victory through attrition. The Flood have shown a preference for close combat. Their less than exemplary motor skills make them poor shots, but they have powerful whipping tentacles that cause great damage at close range. In the melee it is not uncommon for Flood to be destroyed by one another in the madness of combat. The Flood does not worry about such collateral damage since disabled Flood bodies can be reanimated by Infection forms. Dead Flood can only be permanently disabled by being completely destroyed.
===Changes from ''Halo 2'' to ''Halo 3''===
*Brutes may now be infected and transformed into combat forms.
*Melee attacks and plasma weapons were nearly useless against the Flood in ''Halo: Combat Evolved'' and ''Halo 2'', but now can be used effectively against the Flood. The Flood are much more vulnerable to ballistic weapons, as well.
*Pod infectors can mutate humans, unshielded Elites and Brutes into combat forms within seconds (as seen during [[Floodgate]]).
*Pure forms are introduced.
*Combat forms wield weapons far less frequently than in ''Halo: Combat Evolved'' and ''Halo 2''.
*Carrier forms release significantly more Pod infectors when they explode. The explosion does not push the player away much as in ''Halo: Combat Evolved'' and ''Halo 2''. In addition, carrier forms are significantly more difficult to kill with projectile weapons, but they are easily destroyed by plasma weapons.
*Running over Pod infectors while driving a vehicle can result in loss of steering and possibly overturning of the vehicle. Any Pod infectors that are hit by the vehicle will not pop.
*Flood combat forms can no longer commandeer vehicles, like they could in ''Halo 2''.
*Sticking a carrier form with a {{Pattern|Anskum|plasma grenade}} or [[Spike Grenade]] will kill both the carrier form and the pod infectors it carries. This is also true of a [[Kemuksuru|supercombine explosion]].
*Any weapon wielded by humans, Elites, and Brutes will not be dropped when transforming into combat forms and the combat forms will be also unarmed.
*Any humans, unshielded Elite, and Brutes have a low chance to resist or preventing the Pod Infector from infecting them.
*Any Pod infector latching into surviving humans, Elite, and Brutes before the infection can be shot at to prevent infection.


=== [[Halo Graphic Novel]] ===
===Changes from ''Halo 3'' to ''Halo Wars''===
{{Main|The Last Voyage of the Infinite Succor}}
''Significant changes to the Flood's general existence were necessary due to the change of genre from first-person shooter to real-time strategy game. These are as follows.''
It should be noted that the Flood assimilation and behavior in HGN was somewhat different from the games. It is stated by the [[Legate]] that the Flood is mentioned in the holy texts of the Forerunner and they are one of the tests before the Great Journey. He also stated that the Flood were gathering bodies and body ''parts'', giving them to the larger organism which demonstrated some form of intelligence and the general consumption of animal matter. This is similar to the events in Halo 1 where the [[John-117|Master Chief]] boards a Covenant ship to find [[Captain Keyes]]. Cortana notes the Flood are gathering bodies<ref>[[Keyes (Level)|Keyes]], [[Cortana]]: "Look, in the corners. The Flood are gathering bodies."</ref> and it is most likely for the Brain Form that was found on the bridge.  
*Many new forms, such as [[Flood bomber form|bomber forms]], were created for ''Halo Wars'' to expand the "roster" of Flood units. The official explanation is that these forms were created from creatures indigenous to [[Trove]].
*Combat forms have more tentacles than before; they resemble spikes rather than flexible appendages.
*Grunts and Jackals can be converted to combat forms.
*Pod infectors do not explode when they are shot in the cinematics, instead 'deflating' like punctured balloons, though they still explode in gameplay.
*Carrier forms do ''not'' always explode upon death.
*Hero Units that do not have shields will be instantly downed regardless of any remaining HP on them, featuring an instant kill on any enemy infantry without energy shielding.


Also, the rate of infection in the Covenant forces was extremely fast. Within minutes of contact, many Elites and Grunts were mutated quickly into Flood forms. The mutations occurred randomly and were not uniform in any way. Sometimes the Elites were transformed into larger forms than the Combat Form. Other times they developed a Flood-like carapace and on many occasions on the [[Infinite Succor]], the Flood used tentacles as their primary weapon similar to the Gravemind. However, when [[Rtas 'Vadumee]] battled his comrade [[Kusovai]], Kusovai continued to mutate throughout the duration of the battle, suggesting the possibility that over time any infected host can mutate further and further.
===Changes from ''Halo Wars'' to ''Halo Wars 2''===
*The Flood now has a Super Unit, The Abomination.
*New forms are introduced, such as [[Flood spawner form]]s, which can spawn Flood forms on the move and can spawn counter units.
*The Flood now only infects the Squad Leader(Either Jiralhanae or Sangheili depending on the Leader) in a Grunt Infantry unit, while the rest are killed. Marine units are entirely infected, retaining ''Halo Wars'' infection traits for humans.
*Pod infectors can now be killed more effectively with units that deal splash damage.
*The Flood can now infest and commandeer vehicles that are still operational, except Super Units and mechanical units (Mainly Sentinels).
**[[Seeder infector]], a new variety of Infection Form that is able to infect air units, is introduced.
**[[Flood infester form|Flood infestor]], a new kind of pure form, is introduced and specialized in infecting the vehicles.
**Infected mechanical vehicles are immune to Isabel's '''Ghost In The Machine''' Leader Power.
*Turret defenses have had their effectiveness reduced against Pod infectors, even those equipped with anti-infantry upgrades.
**Mega Turret and Siege Turret were exempted from this due to providing splash damage.
*The infection process affects all units. If the Pod Infector units doesn't match its victim (For example, Pod Infestor attacks infantry unit is not match for infection), it will be destroyed or killed. If match, the infection process can be started.
**Any infected units will revert their health back to 100% once the infection process has been completed.
**Infection process can be started once a unit is dying (Red Health).
*New structures for barricades are introduced.
*Pod infector squad size increased 3 to 4 times.
*Biohazard sign appears every time a unit has gotten infected.


In addition, the Flood can instantly absorb knowledge from an infected host into their collective mind. When Rtas' teammates were assimilated, the Flood could open any doors and access any system on the ship right after the teammates were assimilated. When the Flood then assimilated the Legate, the Flood reflected the knowledge of him to Rtas while he was in engineering, mocking him and stating that "The Forerunner not defeat us...what chance you?" It would appear that the threat of the Flood is far greater than previously assumed.
==Production notes==
{{linkbox|gallery=yes}}
[[File:HCE EarliestFlood Concept.jpg|thumb|The earliest extant concept art of the Flood, by Robert McLees.]]
===Miscellaneous===
* The Flood's AI routines in ''[[Halo: Combat Evolved]]'' were originally intended to be "every bit as complicated as the Covenant", but full implementation was prevented by time constraints.<ref>''EDGE'' magazine - September 2003, ''page 109''</ref>


== Trivia ==
===References===
*The name of the Flood is a reference to the biblical story of the [[Wikipedia:Flood myth|Great Flood]]. Some of the parasite's history mirrors the story, in which God destroyed all life except for that which was carried by [[Wikipedia:Noah's Ark|Noah's ark]].
*The Flood bear a strong visual resemblance to an enemy cut from ''[[Marathon|Marathon 2]]'', the “fungal zombie.” These were to be the result of a bioweapon invented in the final days of the ancient S'pht and reused by Robert Blake's forces on Lh'owon, designed specifically to target Pfhor. Both the fungal zombies and Flood were principally designed by [[Robert McLees]].
*The Flood were inspired by Christopher Rowley's [http://www.christopherrowley.net/novels/VangSeries.html The Vang].<ref>[http://www.gamespot.com/xbox360/action/halo3/news.html?sid=6162009&mode=previews&msg_sort=1&cpage=11 ''Gamespot'': Halo - Q&A]</ref>
*An alien virus named "[[tardis:The Flood (The Waters of Mars)|The Flood]]" appeared in a 2009 special episode of the British science fiction television show ''[[wikipedia:Doctor Who|Doctor Who]]''. Although like its Halo counterpart, it is hinted to be intelligent, and is extremely virulent, transforming its victims physically, the Flood found on Mars literally uses water to transmit itself.<ref>''Doctor Who'', ''[[tardis:The Waters of Mars|The Waters of Mars]]''</ref> The previous episode, also a special, involved the alien planet "San Helios", a name very similar to ''Halo's'' [[Sanghelios]].<ref>''Doctor Who'', ''[[tardis:Planet of the Dead|Planet of the Dead]]''</ref>


*There was never a Grunt combat form seen. Probably due to thier stature and cowardess, the Flood seem to only kill the Grunts just for the sake of killing them.
==Gallery==
*The first known Human to be infected by the Flood was a [[UNSC]] [[Marine]] named [[Private Riley]].
<gallery>
*An error in the flood animations in both Halo and Halo 2 is that Combat forms are always seen wielding two-handed weapons, such as the [[Shotgun]], with one hand, and never seem to need to reload them. Just like other [[AI]], the Flood never run out of ammo.
File:HCE EarlyFlood Concept 1.png|Early concept art for the Flood by [[Robert McLees]].
*It could be said that the one responsible for releasing the Flood is [['Qualomee]], because he accidentally told [[Jacob Keyes|Captain Keyes]] of the Covenant "weapons cache". However, it could be argued that the Flood escaped earlier, since the Marines found dead Covenant throughout the installation including an Elite with a hole in its torso. Though, if this were caused by an infection form, it seems odd that it did not remain in the Elite, turning it into a combat form. [[Kappus]] adds that the Covenant worked hard to seal the door where the Master Chief first encounters the Flood. Also, 343 Guilty Spark tells Master Chief that he believes the Covenant responsible for releasing the Flood.
File:HCE EarlyFlood Concept 2.png|Early concept art for the Flood by Robert McLees.
*You can kill the infection form with a sword and you wont lose any of its battery power.
File:HL Origins BigFlood.png|A large unidentified Flood form seen in ''[[Origins|Origins I]]'' from ''[[Halo Legends]]''.
*In halo 2, if you destroy both arms of the combat form, for some reason, they will walk in one direction very slowly then explode leaving the infection form there. The reason this happens is unknown, but it might be a process for the infection form to get out of a damaged body.
File:HL Origins BigFlood2.png|An up-close view of the same Flood form in Origins I.
*It is possible for the [[flood]] to wield [[sniper rifles]] and [[beam rifles]], but they never fire them,they only melee with them instead.The only time they would fire them is when you kill the flood wielding the weapon it will fire stray shots as any character would.
File:HINF-FloodFX.jpg|A Spartan-IV with Flood cosmetics in ''[[Halo Infinite]]''.
*In [[Halo 2]] you can instantly kill a [[Combat Form]] by shooting at its chest, where the tendrils come out. You will know that you have a perfect shot when the cross hairs turn solid bright red and a small red dot appears in the center. This destroys the Infection form, leaving only an empty shell of a corpse. You may have to do this multiple times, though, even though it may appear like you have a perfect shot. This also can be done with [[Sniper Rifle|Sniper Rifles]] or [[Beam Rifle|Beam Rifles]], but it is very hard to do so. [[Shotgun|Shotguns]] do not work, since they kill it anyway. This is best done on [[Sacred Icon (Level)]] with a [[Battle Rifle]]. In the later stages of [[Halo 2]] some flood combat forms start utilizing the [[Energy shield]] from their victim. This makes them much more dangerous, as it takes roughly double the damage to take them out and needler ammo doesn't go near them.
File:HINF-ForgeFlood1.jpg|A [[Forge]] Flood palette on [[Oasis]].
File:HINF The FloodLogo.png|Icon for the Flood manufacturer in ''Halo Infinite''.
</gallery>


== References ==
==List of appearances==
{{col-begin}}
{{col-2}}
*''[[Halo: Combat Evolved]]'' {{1st}}
*''[[Halo: The Flood]]''
**''[[Halo: The Flood#2010 bonus content|Adjunct]]'' {{Mo}}
*''[[Halo: First Strike]]''
*''[[Halo 2]]''
**''[[Conversations from the Universe]]'' {{Mo}}
*''[[Halo Graphic Novel]]''
**''[[Breaking Quarantine]]''
**''[[The Last Voyage of the Infinite Succor]]''
*''[[Halo: Ghosts of Onyx]]''
*''[[Iris]]''
*''[[Halo 3]]''
**''[[Bestiarum]]''
*''[[Halo: Contact Harvest]]'' {{Mo}}
*''[[Halo Wars]]''
*''[[Halo 3: ODST]]'' {{MCCo}}
*''[[Halo Legends]]''
**''[[Origins]]''
*''[[Halo: Evolutions - Essential Tales of the Halo Universe]]''
**''[[Human Weakness]]''
**''[[Soma the Painter]]''
**''[[The Mona Lisa]]''
**''[[Wages of Sin]]''
*''[[Halo: Blood Line]]'' {{Mo}}
*''[[Halo: Reach]]''
**''[[Data pads]]'' {{Im}}
*''[[Halo: Cryptum]]''
*''[[Halo: Glasslands]]'' {{Mo}}
*''[[Halo Living Monument]]'' {{Mo}}
*''[[Halo: Combat Evolved Anniversary]]''
**''[[Terminal (Halo: Combat Evolved Anniversary)|Terminals]]''
*''[[Halo: Primordium]]''
*''[[Halo 4]]''
**''[[Terminal (Halo 4)|Terminals]]''
*''[[Halo: Silentium]]''
*''[[Rebirth]]'' {{Mo}}
*''[[Halo: Spartan Assault]]''
*''[[Halo: Escalation]]''
{{col-2}}
*''[[Halo: Mortal Dictata]]'' {{Mo}}
*''[[Halo: Broken Circle]]'' {{Mo}}
*''[[Halo: Nightfall]]'' {{Mo}}
*''[[Halo 2: Anniversary]]''
**''[[Terminal (Halo 2: Anniversary)|Terminals]]''
*''[[Halo: New Blood]]'' {{Mo}}
*''[[Halo: Hunters in the Dark]]'' {{Mo}}
*''[[Halo: Last Light]]'' {{Mo}}
*''[[Halo 5: Guardians]]'' {{Mo}}
*''[[Halo: Shadow of Intent]]'' {{Mo}}
*''[[Halo Mythos]]''
*''[[Halo: Fractures]]''
**''[[Defender of the Storm]]''
*''[[Halo: Tales from Slipspace]]''
**''[[Something Has Happened]]''
**''[[Dominion Splinter]]''
*''[[Halo Wars 2]]''
*''[[Halo: Envoy]]'' {{Mo}}
*''[[Halo: Fireteam Raven]]''
*''[[Halo: Renegades]]'' {{Mo}}
*''[[Halo: Outpost Discovery]]''
*''[[Halo: Shadows of Reach]]'' {{Mo}}
**''[[Sacrifice]]'' {{Mo}}
*''[[Halo: Point of Light]]'' {{Mo}}
*''[[Halo: Divine Wind]]'' {{Mo}}
*''[[Halo Infinite: Memory Agent]]''
*''[[Halo Infinite]]''
*''[[Halo: The Rubicon Protocol]]'' {{Mo}}
*''[[Halo: Outcasts]]'' {{Mo}}
*''[[Halo: Saturn Devouring His Son]]''
*''[[Halo: The Television Series]]''
**''[[Unbound]]'' {{Mo}}
*''[[Halo: The Television Series Season Two]]''
**''[[Sanctuary (TV Series)|Sanctuary]]'' {{C|Visual allusion}}
**''[[Thermopylae (TV Series)|Thermoylae]]'' {{C|Visual allusion}}
**''[[Halo (TV Series)|Halo]]''
*''[[Halo: Epitaph]]''
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<references/>
==Sources==
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{{Flood}}
{{Flood}}
{{Organizations}}


[[Category:The Flood]]
[[Category:Flood| ]]
[[Category:Factions]]
[[Category:Host species| ]]
[[Category:Sapient species]]

Latest revision as of 15:23, October 13, 2024

This article is about the life form. For other uses, see Flood (disambiguation).
Flood
A render of a collection of Flood forms, as they appear in Halo: Fireteam Raven.
Biological overview

Latin name:

Inferi redivivus

Diet:

Parasitic

Physical information

Avg. height:

Variable (depending on host)

Avg. weight:

Variable (depending on host)

Distinctions:

Variable

Sociocultural information

Homeworld:

Extragalactic

Technology level:

Variable

Notable individuals:

Also known as:

 

"One single Flood spore can destroy a species."
— Rtas 'Vadum[2]

The Flood, designated as LF.Xx.3273 by the Forerunners (Latin Inferi redivivus[3] meaning "the dead reincarnated"[4]) and referred to as the Parasite[5] and the infection[6] by the Covenant, is a species of highly virulent parasitic organisms that reproduce and grow by consuming sentient lifeforms of sufficient biomass and cognitive capability. The Flood was responsible for consuming most of the sentient lifeforms in the galaxy - including the vast majority of Forerunners - during the Forerunner-Flood war in ancient past, prompting the activation of the galaxy-sterilizing Halo Array in 97,445 BCE.[7]

Originating as a corrupted form of the galaxy's most ancient custodians (the Precursors)[8] the Flood reproduces and spreads by infecting other organisms, hijacking their bodies and nervous systems in order to transform them into one of many specialized forms. Through infecting sentient lifeforms, the Flood assimilates their memories and intelligence, and the species becomes collectively more intelligent as a result. Because of their frighteningly rapid growth rate, as long as enough hosts of sufficient biomass and intelligence are available, the Flood is effectively unstoppable.

All Flood forms share a single consciousness, which becomes progressively more intelligent and sophisticated with every sentient host that is assimilated. When enough hosts have been infected, the Flood consciousness becomes a Gravemind - a self-aware, highly intelligent entity that strategically commands all Flood forms in an outbreak. Due to this group consciousness, the Flood as a whole is more accurately described as a single, transsentient superorganism rather than a collection of individuals directed by one or more controlling beings. In the absence of a Gravemind, the Flood possesses only basic coordination; they are incapable of forming complex strategies, and focus solely on attacking and infecting nearby sentient organisms. This stage of a Flood outbreak is known as the Feral Stage. At this stage, the Flood seek to create an assembly of biomass, calcium, and nervous system reserves, which leads to the creation of a Flood hive and the beginnings of a proto-Gravemind. The nature of the Flood's collective consciousness is similar to that of a hive mind, the Flood act as a unified entity, with no individuality that is inherent to other species; each vessel of the Flood works tirelessly to aid in the propagation of their species.[3]

The Flood presents a highly variable and unconventional threat in combat, as it can infect and mutate dead or captured opponents into a myriad of deadly forms, effectively growing stronger as it weakens enemy forces. They are widely considered to be the greatest threat to the existence of life (or, more accurately, biodiversity) in the Milky Way galaxy.

History

Origins

"Our urge to create is immutable; we must create. But the beings we create shall never again reach out in strength against us. All that is created will suffer. All will be born in suffering, endless grayness shall be their lot. All creation will tailor to failure and pain, that never again shall the offspring of the eternal Fount rise up against their creators. Listen to the silence. Ten million years of deep silence. And now, whimpers and cries; not of birth. That is what we bring: a great crushing weight to press down youth and hope. No more will. No more freedom. Nothing new but agonizing death and never good shall come of it. We are the last of those who gave you breath and form, millions of years ago. We are the last of those your kind defied and ruthlessly destroyed. We are the last Precursors. And now we are legion."
— The Gravemind to Catalog in the endgame of the Forerunner-Flood war.[8]

Over ten million years before present day, a civilization of supremely advanced beings known as the Precursors thrived in the cosmos. Having assumed the Mantle - the responsibility for the guardianship of all life in the galaxy - they seeded numerous worlds with life, bringing forth a wide range of sentient species across the galaxy. Under their guardianship, numerous intelligent species developed and flourished, including the Forerunners and humanity. The Precursors would eventually choose humanity to be the rightful successors to their Mantle, while judging the Forerunners unworthy. Refusing to accept their creators' judgment, the Forerunners staged a massive rebellion against the Precursors.[8]

The ensuing conflict ended with the Forerunners successfully wiping out all but a few Precursors.[9] The few Precursors that escaped from (or were spared by) the Forerunners either went into suspended animation or transmuted themselves into dust that would regenerate into their past forms at a later time. However, over time the dust became defective, creating sickness, disease and biological mutations in other organisms that came into contact with it. With this new form — the earliest stage of the Flood — the Precursors vowed that none of their creations would rise against them again.[8] A significantly mutated Precursor, one that would later be known as the last surviving one of its kind,[10] was sealed in a Precursor stasis capsule and placed in a small, ravaged planetoid at the galaxy's edge around 9,000,000 BCE.[11] This being, later known as the Primordial, would eventually reveal the nature of the Flood to those willing to know the truth.[8]

First outbreak

Antecedents

The Flood, as it is known in modern times, first appeared in the galaxy some time prior to 107,445 BCE, after ancient humans came into contact with the Precursor dust. This fine, desiccated powder had been stored in millions of glass cylinders placed aboard automated starships,[12] bound toward the margins of the Milky Way galaxy from the direction of the Large Magellanic Cloud.[13] Forthencho came to believe that the Primordial, despite its hibernating state at the time, may have been responsible for remotely triggering the movement of these ships.[14] However, over a span of ten million years, the substance had slowly deteriorated, along with the ships themselves failing.[12]

The crashed remains of these ships were discovered by ancient humans on both inhabited and deserted worlds near the border of intergalactic space, a region of the galaxy to which humans expanded early on in their history. Inside the ships, humanity found the capsules filled with powder, which their tests told them consisted of inert, short-chain organic molecules of unknown purpose.[12] Humanity studied the material and deemed it to be harmless, but noted that it had psychotropic effects on lower animals. Small dosages were administered to Pheru, popular domestic animals among humans and San'Shyuum, which resulted in more docile behavior.[15] Unbeknownst to the humans or San'Shyuum at the time, the powder was altering the genetic code of the Pheru, manifesting as improved behavior while slowly changing other aspects of their genome.[12]

Emergence

After the humans had exposed the Pheru to the powder for several centuries, more disturbing changes appeared. Pheru treated with the powder began exhibiting furry growths and fleshy protrusions, which other Pheru were compelled to consume. Shortly afterward, the Pheru began suffering widespread abortions and genetic deformities, and many were euthanized or released into the wild. Soon afterward, the humans and San'Shyuum who had been in the presence of the altered Pheru began to exhibit some symptoms of the powder-induced "disease". Affected on a psychological level by the strange illness, infected victims began to consume the sick Pheru, and anything they touched became infected while parts of themselves that were discarded, such as tissue and limbs, also became vectors for the disease. The afflicted then moved on to cannibalism and sacrifice of the infected, inducing a sacrificial individual to grow to prodigious size and then forcing uninfected individuals to consume them while they were still alive. During this time, the infection passed from human to San'Shyuum and human to human, but rarely San'Shyuum to human. The infected humans and San'Shyuum proceeded to spread the disease into other systems, which furthered the scope of the Flood outbreak.[12]

At this point, the Flood infestation, which had previously been predominantly a mind and behavior-altering affliction with only relatively minor physical changes, began to reshape its victims into a variety of forms designed to gather victims, engage in combat, and to rapidly consume useful biomass and other resources.[12]

Spread and withdrawal

The infection quickly spread through both human and Forerunner space. Attempts to quarantine infected worlds and systems failed, leading to the Flood soon infecting hundreds of worlds in fifteen systems.[12] In an effort to halt the Flood's continuing spread, human fleets began destroying Flood-infested Forerunner ships and cleansing Forerunner planets where Flood infestations were detected. Forthencho, the commander of the human fleets, chose not to warn the Forerunners in advance of these actions to prevent giving the Flood time to expand.[16] In turn, many Forerunners believed that the Flood was a fiction meant to hide humanity's expansionist aims. Ultimately, the Forerunners interpreted humanity's tactics as acts of aggression, leading to the Human-Forerunner wars.[12]

With humanity severely weakened from the Flood infection and now fighting a two-front war, the Forerunners were surprised at the ease of their earliest victories against the humans. However, apparently without warning, the Flood began to retreat from its assault on humans. Forerunner ships found healthy human worlds within sectors of Flood infestation, leading Forerunners to conclude that humanity had developed a natural resistance, immunity or cure to Flood infection.[17] However, no immunity or cure to the Flood existed; the Flood had chosen to stop infecting humans.[18] The Didact later speculated that the Flood's intent was to prevent humanity's extinction at the hands of the Forerunners by making it appear as if humans held a solution to Flood infection.[19]

At this time, the Forerunners also first came into contact with the Flood. The parasite devastated many of their fleets, before its deliberate recession from the galaxy tipped the scales in the Forerunners' favor, allowing them to sterilize any Flood infestation they came across in several thousands of worlds across the galactic border.[17] While recognizing the legitimate concern of a Flood resurgence in the future, the Forerunners had difficulty extracting any intact specimens of the parasite for study; approaching the threat with appropriate caution, the Warrior-Servants directly cauterized any active infestations they encountered. From the scattered remains recovered for analysis, the Forerunners discovered that AIs that had been in contact with the Flood had been corrupted beyond repair by what the Forerunners would come to know as the logic plague, essentially the data equivalent to Flood infection.[17] No fully intact specimen of the Flood could be found among the Pheru, humans, or San'Shyuum, as they had methodically destroyed any sign of infection, including all the original carrier vessels that they had discovered.[20] Any remaining Flood were believed to have fled the galaxy aboard infected ships,[20] and the Flood was not seen again for nearly ten thousand years.

While they managed to regain some of their strength once the Flood began to recede, humanity still stood against the Forerunners in the later stages of the conflict. Their campaign against the Flood had worn their resources thin, while the Forerunners led by the Didact had managed to entrench their forces in advantageous positions which proved too powerful for humans to hold against.[20] System by system, the humans fell back to their capital, Charum Hakkor, where they were finally defeated. After losing the conflict, the Forerunners devolved humanity and exiled them to their homeworld while erasing all signs of human achievement from the galaxy.

Interim

Due to the lack of evidence, most Forerunners were unaware of the existence or of the true nature of the Flood, and many dismissed it as a simple excuse for humanity's aggressive expansion before and during the initial arrival of the Flood. However, several high-ranking Forerunners were fully informed of the threat, and in secret began developing countermeasures should the Flood ever return. This became the source of a major political strife, with the Builders proposing the construction of an array of superweapons - Halos - while the Warrior-Servants led by the Didact championed a more conventional strategy of carrying out precision military campaigns from planet-sized fortresses known as shield worlds. The Didact eventually lost to the Builders and the entire Warrior-Servant rate was marginalized, with their fleets and forces disbanded or forced to submit under Builder control. This proved to have been a grave mistake, as the Forerunners found themselves ill-prepared to fight off the Flood after the parasite returned to the galaxy.

Realizing that the Flood stood a significant chance of wiping out all sentient life and the prospect of using the Halos to stop them, the Lifeworker race of the Forerunners began a "Conservation Measure", researching the Flood while cataloging and preserving the galaxy's species aboard a number of facilities related to the Halo Array, mainly an extragalactic installation known as the Ark.

Forerunner-Flood war

Main article: Forerunner-Flood war
Artwork from the Terminals depicting the Flood moments before the firing of the Halo Array.
A Flood-infested Forerunner city, moments before the Halo Array's activation.

"It fed on intelligent life and, in doing so, became ever more intelligent itself. The Flood was unique; it used their very strength against them."
— Cortana on the Forerunners' war against the Flood.

The Flood reappeared as a viable threat in the galaxy when they were first encountered by the Forerunners on the planet Seaward. Initially, the Forerunners severely underestimated the potential threat that this parasitic lifeform posed, and used tactics more suited to disease control than actual warfare. By the time the infection had begun to spread, the Flood had become a much deadlier and more intelligent foe. While at first the Flood had demonstrated a lack of even rudimentary strategic ability, the parasite quickly began to amass biomass and create Graveminds, Flood forms that embodied a collective intelligence for the Flood species, and that could coordinate the Flood swarms in attacks against the Forerunner fleets using all Flood forms within the vast range of their seemingly telepathic control.[21]

A common tactic used by the Flood was to use huge numbers of unarmed civilian vessels to penetrate planetary defense groups, bypassing the Forerunners' orbital fleets, and infesting the planet and its populace below. The sheer numbers of Flood forms on the planets overwhelmed Forerunner ground forces. Billions of forms were dedicated to planetary assaults, and every member of the Forerunner population was a potential Flood host. These tactics were demonstrated in the Flood's attacks on numerous worlds, including LP 656-38 e and DM-3-1123b.[21] In desperation, the Forerunner armada was ordered to commence full planetary bombardment on infested worlds, although at a great cost - many Forerunners were unable to be evacuated before the bombardment commenced, leaving the Forerunner military with shallow victories against the Flood. In events where the naval garrisons were unable to commence bombardment, major Forerunner population centers were seen to activate localized weapons of mass destruction, effectively committing mass suicide, but stopping the growing infestation.[21]

Thereafter, the Forerunners realized that conventional naval tactics were unable to stem the growing tide of the Flood, and decided to pin their hopes of defeating the Flood on more extreme measures. While the Forerunners initially deployed robotic Sentinel drones to fight against and contain the Flood, almost immediately afterwards, the Forerunner Fleet Command contemplated "Premature stellar collapse": using naval battle groups to send stars into supernovae that would engulf their planetary systems and prevent any possibility of Flood infection and destroy all Flood present in the system.[21] As a further setback to the Forerunners, an extended dialog with the Primordial provoked Mendicant Bias, the Contender-class AI in charge of the Forerunner defense, to succumb to the Flood's logic plague and turn its vast resources against its masters. While the Primordial's physical body was eventually destroyed, it's consciousness was either consumed or transferred to the Gravemind, granting the Flood previously unprecedented intelligence and power.[22]

Final decade

The Flood grew exponentially, spreading from system to system with the intent to infect the Forerunner population, not to wage war with Forerunner battle groups which were poorly adapted to such strategy. As the Flood spread further, entire planetary biospheres were converted to Flood biomass. These planet-sized key minds, locuses of Gravemind intelligence that began to outcompete the Forerunners through raw computational might alone.[23][24] The logic plague continued to take on more adaptive forms, defying all defenses the Forerunners devised and eventually spreading itself between AIs and data networks. As the Forerunners relied significantly on AI automation in combat, this severely hindered Forerunner military efforts against the Flood. Adding to these difficulties was the disturbing revelation that the Flood was capable of manipulating Precursor technology: towards the close of the war, the Flood began to appropriate Precursor megastructures such as the star roads, wielding the previously-inert strands to pummel Forerunner planets and naval forces. This ability was a result of the Flood's Precursor origins; being corrupted Precursors, they could tap into neural physics and activate Precursor artifacts. The proliferation of high-level key mind nodes also enabled the Flood to use the Precursors' neural physics-based superluminal transit, which in turn encumbered Forerunner slipspace travel. Towards the end of the infestation, Forerunners noted that space itself seemed to become hostile and unpleasant from mere perception, an artefact of Flood-based Neural Physics. [23]

The Halos' firing

Main article: Great Purification

The Forerunners employed countless measures to contain the Flood outbreak, all of which failed. Finally, the Forerunners were forced to a Pyrrhic solution: using an array of seven ring-shaped megastructures collectively known as the Halo Array as superweapons, the Forerunners would purge the galaxy of all sentient life. Because the Flood were parasitic, and their survival was directly linked to the presence of potential hosts (and in particular hosts capable of sentience) the Forerunners reasoned that eliminating all potential Flood hosts would render the parasite unable to grow and spread, and eventually cause it to starve to death. Additionally, as a result of the Master Builder's test of a Halo at Charum Hakkor, it was found that the Halo Effect unintentionally caused the complete destruction of artefacts of Neural Physics. This was likely a byproduct of how the Halo Effect was designed to destroy any and all forms of neurological structure subjected to it.[25] The decision led to political battles and even a civil war, but after exhausting every countermeasure, the Forerunners activated the Halos in 97,445 BCE.

As a result of the firing of the seven Halo installations, the surviving Flood were successfully contained and eventually died out. The only surviving specimens were contained in state-of-the-art, high-security Forerunner research facilities, such as the Halo installations, certain Shield Worlds, and the Threshold gas mine. All Precursor artifacts and structures, composed and maintained entirely using Neural Physics were also destroyed and left no trace.[26] Many sentient species throughout the galaxy (including humanity) survived the firing of the Halo Array, having been safely relocated beyond the galaxy by the Forerunners on the Ark.

During their expansion throughout the Orion Arm, the Covenant would find and occasionally release the Flood, employing Zealots and their Temple Wraiths to contain the outbreaks.[27][28]

Outbreak on Trove

Main article: Battle of Trove

Thousands of years after the end of the Forerunner-Flood war, the human ship UNSC Spirit of Fire jumped from Arcadia to follow the signal of Professor Anders, who had been captured by the Covenant. They were led to an unknown planet, which happened to be a Forerunner shield world. Containment measures on this planet had failed catastrophically, and the entire surface of the megastructure was choked with Flood biomass. Ground forces were deployed to investigate the presence of unknown hostiles sighted by the ship's radar. After fighting their way through Covenant forces, Sergeant Forge and several Marine squads were attacked by the Flood.

After successfully defeating the Flood forces, the UNSC forces created a firebase in the area that Anders' signal appeared to be coming from. The humans eventually tracked the signal to a massive Proto-Gravemind attached to several Flood colonies. The Proto-Gravemind had duplicated Anders' signal, possibly in hopes of infecting the humans and spreading beyond Etran Harborage. It was discovered that killing a Flood colony weakened the Proto-Gravemind and SPARTAN-II Red Team, with help from the firebase, destroyed the Proto-Gravemind.

The Flood responded by launching a massive attack on the Spirit of Fire's Alpha, Bravo and Charlie Platoons to avoid them returning to the ship. Despite massive Flood resistance, forces from the Spirit of Fire were able to rescue the platoons before their firebase was overrun by the Flood.

With the Proto-Gravemind dead, the smart-AI Serina was able to reacquire Anders' actual signal, which led to an ocean under which a docking port leading to the interior shield world was located. During the Spirit of Fire's descent, the outer surface of the ship was infested by the Flood. With the aid of Sentinels and "cleansing rings" controlled by the installation's AI, the Spirit of Fire crew were able to cleanse the ship and proceed into the shield world.

Within, the UNSC forces discovered that the Flood had managed to infest the interior of the shield world as well, though not to as massive of a degree as the surface planet itself. While rescuing Anders, Forge had to save her from three pod infectors and ran into some trouble with Flood forces on his following missions, though not to the massive extent they had been seen on the exterior planet.

In order to prevent the Covenant from getting their hands on Etran Harborage's fleet of Forerunner Sojourner-class dreadnoughts, a plan was created to use the reactor from the Spirit of Fire's Shaw-Fujikawa Translight Engine to induce the shield world's artificial star to go supernova. The plan was a success and Etran Harborage was destroyed. The explosion obliterated the Flood on the planet, along with the Covenant forces and the Forerunner fleet.[29]

Brief recurrence

Main article: Outbreak on the UNSC Spirit of Fire

On January 1, 2537, six years after the events on Trove, a Flood outbreak was initiated aboard the UNSC Spirit of Fire by a Pod infector that had managed to remain hidden aboard the ship. As the Flood threatened to breach quarantine, Serina awoke Jerome-092 who, with the aid of Professor Ellen Anders, was forced to exterminate all infected crew members, ensuring the survival of the Spirit of Fire and its remaining crew. All traces of the Flood parasite aboard the Spirit of Fire were erased.[30]

Revival of the Flood

Main articles: Battle of Installation 04, Outbreak at the Containment Facility
A matte painting from the terminals depicting the Outbreak at the Containment Facility.
Jacob Keyes and his Marine squad are overwhelmed by the Flood during their expedition into a Flood containment facility.

"The only way to stop the Flood is to starve them to death."
— Cortana to John-117 and 343 Guilty Spark.

Although the first firing of the Halo Array starved the Flood that were at large in the Milky Way Galaxy, Flood specimens were kept in stasis within installations around the galaxy, including Installation 04 and various Forerunner research facilities. This would prove to be a grave mistake, as both humanity and the Covenant stumbled upon one of the abandoned Halo rings thousands of years after the firing of the Array. The UNSC AI Cortana, aboard the UNSC Pillar of Autumn, made what was presumed to be a "blind" jump through Slipspace (in reality, she used translated co-ordinates from a Forerunner artifact found on Sigma Octanus IV) to evade Covenant pursuers. This led both groups to Installation 04, and in turn, the Flood that were kept there in stasis. After the crash of the Pillar of Autumn on the Halo ring, piloted by Captain Jacob Keyes, the surviving crewmembers dug in for what they believed would be a long and brutal guerrilla campaign against the Covenant forces on the ring.

Meanwhile, the pursuing Covenant cruisers, among them the Truth and Reconciliation, amassed their ground forces on the surface, where they soon discovered an underground Forerunner Flood containment facility in the swamps of the ring. Due to the secure nature of the facility, the Covenant came to use it as a fortified base of operations and as a storage area for weapons and ammunition, oblivious to its true nature and purpose.

At some point, either through a malfunction in containment equipment or a blunder on the Covenant's part, the captive Flood forms within the facility's stasis chambers were released and attacked the Covenant forces inside. The Covenant had at least a rudimentary knowledge of what the Flood were capable of, and promptly locked the facility down and evacuated all personnel who survived the incident.

Keyes and his team were unaware of these events, and proceeded to move deep into the facility, ultimately coming across one of the unopened Flood containment chambers. Believing it was a weapons cache of some sort, the team unlocked the door, and were attacked and assimilated by the Pod infectors inside. Upon being informed of Keyes' disappearance, John-117 was deployed to Captain Keyes' last known location on the drop ship Echo 419, proceeding into the facility, only to find out that most of the Marines he had been assigned to rescue, including the Captain himself, had succumbed to the Flood.

These events brought knowledge of the Flood to the UNSC. In a war of losses, this was another grim revelation for humanity.[31]

Voyage of the Infinite Succor

Main article: Skirmish on Infinite Succor

Shortly after their escape from the containment facility, Flood forces took control of a nearby crashed Spirit dropship, the Brilliant Gift and crashed it aboard the Covenant Mjern-pattern agricultural support ship Infinite Succor. The Flood quickly spread throughout the ship, infecting its crew and the animals from the vessel's hunting preserve. The Flood began building a Proto-Gravemind, intending to use the Infinite Succor to flee the Soell system and begin infecting other planets. The ship's Legate, the Minister of Etiology, locked out the Infinite Succor's primary systems while one of the crew's Unggoy managed to send out a distress call. Suspecting human involvement, particularly that of the Demon, Supreme Commander Thel 'Vadamee sent a Covenant Special Operations team led by Rtas 'Vadumee to retake the ship and eliminate the Demon if he were onboard. Quickly uncovering the true threat, Rtas programed the ship to make a slipspace jump into the system's star which would destroy the Infinite Succor and the Flood infestation onboard. Though Rtas succeeded, he was the only survivor.[32]

Assault

A matte painting from the terminals depicting the Battle of Installation 04.
The Flood engages both UNSC and Covenant in their drive to escape Installation 04.

After the Flood's escape, the Covenant forces across Installation 04 were overwhelmed by the Flood's assault. While the Covenant were able to hold the line by using vehicular and air support, most of the weaker Covenant soldiers such as Unggoy and Kig-Yar succumbed easily to the Flood, and the few Sangheili who remained were outnumbered and thus infected. The UNSC, having a much smaller presence on the installation, found out about the Flood's might a short while later, when an ODST force engaged with the Covenant near Alpha Base, was ambushed by the Flood and suffered heavy casualties. However, one of the attacking combat forms, PFC Wallace A. Jenkins, who was one of the Marines in the original expeditionary mission to the supposed arms cache, was infected by a weak and elderly Pod infector whose powers of infection had been dulled by centuries of hibernation, making Jenkins' transformation into a combat form incomplete. Jenkins was still able to resist the pervasive Flood entity in his mind, and although incapable of articulate speech and with a physiologically devastated body, Jenkins was still able to betray the Flood through hand gestures during an interrogation with Major Antonio Silva, informing the UNSC Marine battalion that the Flood planned to assault the UNSC base from a series of underground catacombs underneath their current base of operations. In response to this intelligence, the UNSC moved to secure them, and prevented the Flood from staging another attack.[33]

During these developments, the ring's Sentinels failed to effectively contain the Flood, and because of this, the Covenant and the UNSC sustained heavy losses. The Flood occupied areas of the ring formerly held by the Covenant, although Covenant and Flood forces continued to wage war on the massive ice plains of the ring. Although the Covenant had the benefit of Shade stationary plasma turrets and vehicular/aerial support, the Flood had captured both UNSC and Covenant weapons and because of this had significant numerical superiority.

One of the UNSC personnel captured by the Flood was Captain Jacob Keyes, a brilliant naval strategist of the UNSC Navy. When he was infected, the Flood sensed that he held knowledge that would be especially useful to the Flood. Keyes knew the location of Earth, the birthplace and major stronghold of the human race, a planet with tremendous possible assimilation opportunities for the Flood. Instead of becoming a combat form, he was merged with at least four other victims into a massive, engorged proto-Gravemind aboard the Ket-pattern battlecruiser Truth and Reconciliation, a Covenant warship the Flood had taken over. While he was being incorporated into the proto-Gravemind, the Flood intelligence began to dig through his mind, searching his memories for the location of Earth. Captain Keyes successfully withheld information about Earth by constantly accessing information available on his command neural interface such as his name, rank, and serial number, until John-117 arrived. However, his arrival was too late, for Keyes was already dead, completely consumed by the Proto-Gravemind. It was at that time that John-117 was forced to punch a hole into Keyes' skull to obtain the Captain's CNI transponder.[34]

The Truth and Reconciliation was a major point of contention between the Covenant Special Operations units and the Flood. The Flood had decimated most of the Covenant crew and garrison on board, leaving only isolated pockets of resistance, mostly small numbers of terrified Unggoy and a few Sangheili aboard the vessel. However, below the Truth and Reconciliation, in a series of rocky canyons, the Covenant were much stronger in force, with large numbers of Sangheili augmented by Unggoy, Kig-yar, and Mgalekgolo, fighting off the Flood. The Covenant Council of Masters in charge of operations on Installation 04, however, were terrified by the Flood presence. Because the Covenant battlecruiser had been damaged in a previous space battle with the UNSC Pillar of Autumn, it had been grounded, awaiting repairs to its underbelly and plasma carrying conduits, for at least two days. After the Flood were discovered to be aboard the battlecruiser, the Covenant military leadership sent a Special Operations strike force to the Truth and Reconciliation to commence immediate emergency repairs, prepare it for lift-off into space and neutralize the Flood. These efforts, however, failed, with the Flood eventually retaking the vessel.

Later, the remaining UNSC Marine forces on the ring staged an assault on the Flood-held Truth and Reconciliation, and managed to neutralize most Flood forms on the cruiser in a swift assault, and prepared the battlecruiser for take-off with hastily completed emergency repairs to escape from the ring and return to Earth. However, there was tremendous risk if the Flood-infected vessel traveled to Earth, if there was even one Flood carrier form aboard, "Earth could fall," according to UNSC AI Wellsley, because of the Flood's exponential parasitic potential. Against orders, Marine First Lieutenant Melissa McKay destroyed the warship with whatever Flood forms were still aboard, killing hundreds of UNSC personnel, including an infected Jenkins, but saving Earth and possibly billions of lives from the Flood.[33]

Defeat

The Pillar of Autumn - exploding, with its fusion reactor becoming destabilized, destroying both the ship and the ringworld, to prevent the Installation from being fired and taking the threat of the Flood with it.

The Flood were interested in seizing space-capable vessels, in the hope of spreading the infection away from Installation 04 and across other systems; they attacked the UNSC Pillar of Autumn and cruiser Truth and Reconciliation for this reason. After the UNSC cruiser's crash-landing onto the ring, and after engaging over fifteen enemy warships previously to landing, it had been taken by the Covenant, although a UNSC strike team had managed to temporarily retake the bridge. On the third and final day of the Battle of Installation 04, however, the Covenant were able to gain control of the crashed warship while one of many Covenant patrols around the exterior of the Autumn was ambushed by the Flood and infected; upon the patrol's return to the vessel, the Covenant garrison was attacked by the infected Elite combat forms of the patrol group. The combat forms were able to scatter before they could all be destroyed, and a steady trickle of Flood continued to enter the Pillar of Autumn through vents and openings in the hull, waging a tedious guerrilla battle against the Covenant security force on board. On another occasion, the Flood attempted to enter through cargo containers but were foiled due to the quick thinking of the Unggoy Yayap.

Eventually, a force of Sentinels came on board, overwhelming the dwindling Covenant forces as the Flood began to attack the warship in full force, in the end almost completely obliterating the Covenant presence, and severely limiting the Sentinels' numbers on board. Despite their best efforts and enormous numbers, the Flood, Covenant, and Sentinels were unable to stop John-117 from detonating the wrecked warship's reactors, and escaping in a Longsword interceptor. The resulting thermonuclear detonation from the destabilized fusion reactor cores severed a portion of the ring world, ultimately destroying it and its capability to annihilate all sentient life, and simultaneously killed all remaining Flood on Installation 04, along with any other life forms that were still present on the ring.[33][31]

Outbreak on the Mona Lisa

Main article: Skirmish on Mona Lisa

After the destruction of Installation 04, in October 2552, a human prison ship, the Mona Lisa, arrived in the Soell system and managed to retrieve surviving specimens of the Flood for study from the remains of Installation 04. They began to test the parasite on both human and Covenant prisoners, unaware of its lethality. Soon, the infection broke quarantine and the ship was infested by the Flood. The Flood absorbed the crew and prisoners, then began to build a proto-Gravemind in its engineering section. The infestation ended when Sergeant Zhao Heng Lopez and her marines from the UNSC Red Horse were sent to investigate the ship after retrieving a survivor from the ship. The Flood absorbed many of her marines, leaving her and a single Sangheili to fight for a single-occupant escape pod. The outbreak was most likely contained when the Red Horse destroyed the Mona Lisa using a Shiva-class nuclear missile.[35]

Threshold gas mine

Main article: Raid on the Threshold gas mine
Screenshot of the Flood tadpoles on The Oracle.
Dormant Flood in receptacles housed in the gas mine.

The Flood were also kept in a Forerunner-constructed gas mining facility suspended over Threshold, within at least one of the three arms of the station. A small Covenant special forces strike team, led by Special Operations Commander Rtas 'Vadumee and the Arbiter Thel 'Vadamee, were dispatched by the High Prophets to silence Sesa 'Refumee, the leader of a heretic faction, who was stationed within the mine. The heretics released the dormant Flood within the station in an attempt to halt the progress of the Spec-Ops team through the station.

The heretics' plan backfired as the Flood killed and infected many of their forces while failing to stop the Arbiter and his team. The Flood broke out of the station's arm and spread to the central core of the station, where they overwhelmed the heretics stationed there. Eventually, even the station's Sentinel forces were completely obliterated as the Flood spread to the upper levels of the station. The Flood were unable to escape the station, however, as the Arbiter managed to sever the cable attaching it to the Forerunner support structure far above, sending the gas mine plummeting to Threshold's liquid hydrogen core, destroying it.[36]

Battle of Installation 05

The Flood were in search of the Activation Index, a device which would deprive UNSC and Covenant forces from the privilege of assured destruction of the parasite.
Main article: Battle of Installation 05

On Installation 05, the Flood had been in a continual battle with the Sentinels after the initial firing of the Halos 100,000 years prior. Again, a secure facility was breached and a Flood form managed to escape, thus beginning the battle with Sentinels. While the infestation had been contained to a quarantine zone established outside the Installation's Library, and presumably was severely weakened due to a lack of fresh host bodies, the Flood had managed to keep a Gravemind form alive deep within the cavernous tunnels of the ring. With the arrival of the Covenant and pursuing UNSC forces, the Flood was able to grow its forces considerably and redouble its assault on the Installation's security forces.

The Flood, despite a significant vehicular and infantry presence in the Quarantine Zone surrounding the Library, were unable to deny access to the Covenant and the UNSC. Miranda Keyes managed to slip the In Amber Clad through the Forerunner Enforcer patrols, while the Covenant launched a ground attack, led by Thel 'Vadamee and involving a small team of Spec Ops Elites, using two anti-gravity gondolas to get to the index while attempting to control the gondolas and keep it from slipping into Flood or Sentinel hands. Later, the In Amber Clad was captured by the Flood.[37]

Fall of High Charity

Main article: Fall of High Charity

John-117, who had arrived with Commander Miranda Keyes and Sergeant Johnson, met Thel 'Vadamee in the clutches of the Gravemind, the central controlling intelligence of the Flood hive, created from the combined intellect of tens of thousands of assimilated corpses. The Gravemind attempted to convince Thel, against his own beliefs, and through the use of the captured Monitor, 2401 Penitent Tangent, and the semi-revived and consumed Prophet of Regret to not activate the ring. Even though Thel was still unswayed, although in question of his religion, the Gravemind sent him and John-117 to search for the Activation Index to keep the Covenant from "turning" the "key," transporting John-117 to High Charity, and Thel to Delta Halo's Control Room.[38]

Taking advantage of the distraction that John-117's presence caused, as well as the ensuing Great Schism, the Gravemind took the opportunity to deliver itself, as well as a multitude of Flood underlings, onto the UNSC In Amber Clad and then slipspace jumped the UNSC ship directly inside High Charity, whereupon it immediately crashed into the city's superstructure to deliver its infectious cargo, while a wave of Flood-controlled Pelican dropships delivered Flood forms all over the city. The ill-equipped and distracted populace was no match for the Flood onslaught, and the Covenant Holy City was quickly overrun, with even the Prophet of Mercy falling victim to a Pod infector.[38]

The Flood soon spread to every district, and Flood spores began to infiltrate and overload the ventilation systems within High Charity; the whole city was becoming terraformed into a gigantic Flood hive. The Gravemind regularly rebutted the words of comfort the Prophet of Truth regularly delivered to the city’s population over the comm system, to sadistically instill more panic and unease. John-117, having successfully stayed in front of the wave of Flood running rampant in the streets, stowed away on the Forerunner Dreadnought that was leading Truth's fleet to Earth. Cortana, who had to stay behind to detonate In Amber Clad's engines should the situation arise and Installation 05 be fired, was left in the clutches of the Gravemind, who could potentially learn all possible human knowledge from her, including the entire knowledge directly received from Forerunner systems aboard Installation 04.[39][40]

Earth, the Ark, and the Flood's defeat

A UNSC Marine being infected by the Flood.
Main articles: Outbreak on Earth, Battle of Installation 00

At the conclusion of the Battle of Voi, with the Prophet of Truth activating the Portal and the Jiralhanae fleet escaping to the Ark, a Flood-infested Covenant cruiser appeared out of Slipspace, having penetrated the Elites' blockade of High Charity and crash landed into the city of Voi in an effort to spread the parasite.[41] In order to prevent the Flood from escaping and infecting the entire planet, Commander Miranda Keyes ordered John-117 to detonate the cruiser's reactors and search the ship for Cortana, causing the city of Voi to be destroyed and the Flood there eliminated. As John-117 and Thel 'Vadam fought their way to the ship, the Sangheili assault carrier Shadow of Intent under the command of Rtas 'Vadum arrived and deployed Sangheili task force to aid UNSC marines. John-117, with the help of the Sangheili, was able to battle through the Flood forces, including the newly encountered Flood Pure Forms to reach the crashed cruiser where he found a message from Cortana. Upon this discovery, 343 Guilty Spark arrived, pledging his assistance to the Reclaimers. With the Flood spreading, John-117 boarded a Phantom while the Sangheili went on to glass the infected areas. The remaining Sangheili forces, fulfilling their mission of removing the Flood from Earth, retreated back to the Shadow of Intent, along with Cortana's message and Guilty Spark.[2]

With the assistance of 343 Guilty Spark, Cortana's painful message was repaired, revealing a solution to the Flood menace on the Ark. To Lord Hood's disapproval, Miranda Keyes led UNSC forces allied with Sangheili forces through the Portal at Voi to the Ark.[2]

On the Ark, while the Sangheili and human forces were attempting to infiltrate Forerunner shield control centers and lower the shield surrounding the Ark's Citadel, the infested High Charity made a slipspace jump and smashed into the Ark, releasing the Flood onto the superstructure. The Gravemind then made a brief alliance with the Master Chief and the Arbiter, Thel 'Vadamee, in order to stop the Prophet of Truth from activating the Halo Array, providing Flood backup to their assault on the Citadel. Once Truth was dead, however, the Gravemind treacherously ordered the Flood to turn against the two heroes. The Chief and the Arbiter escaped, however, with some assistance from the Sentinels, who were able to help subdue the Flood with ease. As they escaped the Flood, the Chief and the Arbiter discovered a replacement Halo ring rising from the Foundry and realized that Cortana's plan to stop the Flood was to fire the new installation at the Ark, an act that would sterilize the local infestation while leaving the Milky Way galaxy unharmed.[42]

John-117 stepping in Flood biomass as he enters the Flood infested High Charity.

In order to fire the new Halo, the Master Chief ventured into the derelict and wrecked High Charity, now a massive Flood hive, where the AI was still held prisoner by the Gravemind. Facing increasing Flood forces and telepathic demands from the Gravemind, the Master Chief was able to rescue Cortana who still possessed the Index from Installation 04. In order to buy time to escape and to delay the Flood, the Master Chief detonated High Charity's reactors, destroying the hive and severely injuring the Gravemind while the Chief, Cortana and the Arbiter escaped on a Pelican.[43]

The Chief took Cortana and the Arbiter to the new Halo ring, joined by Sergeant Avery Johnson in the UNSC Forward Unto Dawn while the surviving humans and Sangheili retreated through the portal to Earth. However, the Gravemind survived High Charity's destruction and began to rebuild itself on the ring while the Flood invaded, attempting to stop the new ring from being fired. Aided by Sentinels controlled by 343 Guilty Spark, the Chief, Johnson and the Arbiter were able to reach the Control Room. However, as firing the ring prematurely would destroy it and severely damage the Ark, Spark turned on the three, mortally wounding Johnson before the Chief apparently managed to destroy him. Cortana was able to use the Index from Installation 04 to fire Installation 08 which killed most of the Flood forms on the Ark along with the Gravemind. However, in its final moments, the Gravemind addressed the Master Chief and the Arbiter telepathically, warning them that their victory would only slow—not stop—the Flood. As predicted by Spark, the installation was destroyed by the firing and the Ark suffered heavy damage. The portal collapsed as the Chief and the Arbiter escaped on the Forward Unto Dawn, leaving the Master Chief stranded in deep space.[44]

Aftermath

"You've never heard of a major Flood outbreak since the war because of what they do, not because the ugly suckers haven't broken out of their cages since then."
— Gabriel Thorne's comment on SPARTAN-IV personnel volunteering to join Flood containment teams.[45]

Even though the Gravemind was destroyed, and all Flood on the Ark apparently eradicated, the Flood still existed on the other Halo rings (excluding Installation 05 which was sterilized by Sangheili forces) and other remaining Forerunner installations. Even though Path Kethona was sterilized by Omega Halo near the end of the Flood war, given the Flood's extragalactic origin, it was possible that more pockets of Flood existed outside of the galaxy - or more disturbingly, that other, more distant galaxies may have fallen to the Flood.

Though the UNSC kept the Flood's existence concealed from the general population, it did make preparations for the possibility of the Flood's return. The new Spartan branch was designed as a first line of defense against the Flood, with the SPARTAN-IV complement of the UNSC Infinity running simulations depicting the possible return of the Flood. This is realized through both War Games exercises and the interactive TACSIM.[46] Some Spartans volunteered to join Flood containment Fireteams dedicated to going up against Flood outbreaks, with several outbreaks occurring following the end of the war and being successfully contained due to their actions.[45] Teams were deployed to quarantine any areas of potential Flood contamination during the UNSC's initial forays onto Halo Installation 03,[47] though it is unclear whether the parasite was actually encountered. The UNSC has otherwise seen the development of the Hazops Cyclops dedicated to fighting the Flood threat, and the "Flood" variant of the VISR - given to Spartan personnel involved in Flood containment duties.[48] The UNSC includes a briefing on the Flood in the Spartan Field Manual, wherein it details protocols such as Emergency Contact Protocol Upsilon and the CORRUPTER cryptonym - to be declared in the suspected presence of a Flood outbreak and upon infection of a Spartan, respectively.[45]

Three days after the Battle of Installation 00, with some Flood spores having survived the firing of Installation 08 and the destruction of High Charity's reactor, the Ark's monitor 000 Tragic Solitude had the ruined Covenant capital city of High Charity sealed off with a containment shield and the Ark's Sentinels raze the perimeter and modify the refugia nearby so that no sentient life would survive in the vicinity, thus depriving the Flood of new host bodies were they ever to escape. Tragic Solitude also set a battalion of Sentinels on patrol around the perimeter, not only to keep the Flood contained, but to defend against any future outsiders who might try to enter the wreckage and thus unleash the Flood.[49]

Nightmare on the Ark

"I told you NOT to go inside. FOR GOOD REASON!"
— Atriox to Voridus[50]
A Flood tank form overwhelms a Warthog.
Main article: Outbreak on Installation 00

In early June 2559, after the loss of the Enduring Conviction, the Jiralhanae brothers Pavium and Voridus were tasked to salvage the wrecks around High Charity for weapons to use in the conflict with the humans of the UNSC Spirit of Fire. The Banished leader Atriox, aware of the potential danger of the Flood having survived in the city, gave specific orders to only scout the shell and not to go inside lest surviving Flood be disturbed. However, Voridus, like many of the Banished, did not take the threat of the Flood seriously, believing it simply be another lie of the Prophets. Ignoring the warnings of both Atriox and his own brother Pavium, Voridus deactivated the Ark's Sentinel defense network and used a Barukaza Workshop Scarab to burn a hole in the containment shield around the former Covenant holy city. When Voridus' troops entered the wreckage to salvage it, they were ambushed by thousands of Flood infection forms lurking within High Charity which were then set loose upon the Ark. With the Sentinel defense network down, the Flood were left unhindered by the Ark's defense systems.[51]

Pavium quickly assembled a force to rescue his brother, defending against overwhelming Flood forces before finally retreating to meet up with Voridus at a Banished drill site near a Forerunner archive that contained a control terminal for the Sentinel defense network.[52] As the area was honeycombed with fissures filled with unstable explosive power, Pavium managed to use Banished Salvage Drills to start a chain reaction to burn away the majority of the Flood in the area, allowing Voridus to access the archive.[53] Inside, despite heavy Flood resistance, Voridus and his troops managed to reactivate the Sentinel defense network which began to cleanse the Flood.[54] However, by this time, the Flood had created a massive Proto-Gravemind in the area near High Charity that was in danger of becoming a full-fledged Gravemind while the threat from the Flood forced Atriox to pull back from positions he had established to fight the humans.[50] This Proto-Gravemind retained a measure of the deceased Gravemind's intellect, but the Gravemind's new vessel had to feed and spread in order for the Gravemind to fully manifest once again.[55] Voridus, Pavium and Banished forces, with the help of the Ark's defenses, in particular a Retriever Sentinel, managed to kill the Proto-Gravemind before it could become a Gravemind.[50]

With the Proto-Gravemind dead, the Flood threat was greatly diminished. Atriox ordered Pavium and Voridus to clean up their mess and the Banished joined a force of thousands of the Ark's Sentinels in containing the Flood outbreak and sealing off High Charity once more.[50] The Banished and the Sentinels ultimately ended up "scouring" the Flood from High Charity's ruins.[56]

Flood forms

The Flood is composed of a variety of specialized forms, each filling a different role in combat. The most basic Flood forms are the infection forms, which come in various types and infect hosts by hijacking their bodies and nervous systems.[57] Infected hosts then undergo a rapid series of transformations, becoming lethal combat forms. Combat forms are used to subdue and secure additional hosts for infection, and once they have outlived their usefulness as warriors, they transform into carrier forms, spreading the Flood by spreading more infection forms.

Once the Flood has amassed enough bodies to create a Gravemind, it is capable of generating "pure" forms, which are created solely from assimilated nutrients and biomass. The pure forms observed during the outbreak in late 2552 were able to spontaneously mutate between three configurations: Stalker, Tank and Ranged forms, although in practice the true extent of their mutability is impossible to effectively catalog.[58]

The Flood at this point relegate the combat forms to defensive purposes or as additions to the biomass, calcium, and nervous system reserves of the Flood hive, and rely increasingly on the pure forms as their primary instrument of infantry force. However, in order for the pure forms to fully replace the combat forms, the pure Flood population must be large enough and in a state of equilibrium, which is in a constant state of flux depending on Flood gains and losses. Because of this, it is common for the combat forms to serve alongside the pure forms, in addition to the infection and carrier forms, as the acquisition of hosts and technology would likely outstrip the speed in which the Flood can generate pure forms.

Spores

Main article: Flood spore

Spores are minuscule airborne Flood forms that serve to spread infection and suffocate potential hosts who inhale them.

Infection forms

Infection forms are the most basic category of the Flood parasite, consisting of several varieties of both ground- and airborne Flood forms.[57]

Tadpoles
Screenshot of a Flood tadpole on The Oracle.
A Flood tadpole in a tank.
Main article: Flood tadpole

Pod infectors undergo a tadpole phase of development before growing into full physical maturity.[59]

Pod infectors
Main article: Pod infector

These are among the most basic of Flood forms. They are tasked with seeking hosts, then infect them that mainly on the ground.

Seeder infectors
A Seeder infector.
Main article: Seeder infector

These are airborne forms of Pod infectors that mainly tasked with infecting airborne creatures.

Pure forms

These forms are the natural flood form and are produced depending on need and amount of biomass available.

Tank form
Main article: Pure form

These large tanking forms can only be produced when a Flood outbreak has amassed considerable amounts of biomass and formed a Gravemind. These forms can take and deal massive amounts of damage.

Ranged form
Main article: Pure form

These flood forms can only be produced once a Flood outbreak has obtained enough biomass to form a Gravemind. These forms attack via bony spikes shot from it's back area, however is immobile.

Stalker form
Main article: Pure form

These pure forms can only be produced if a Flood outbreak has gathered a large quantity of biomass to creat a Gravemind. These forms are very fast and agile.

Infester Forms
Artwork of the Flood infester form. In the Halo Encyclopedia (2022 edition), this art is incorrectly used to depict the Flood spawner form.
An Infester form.
Main article: Flood infester form

These forms attack and infect vehicles.

Carrier forms

These forms of the flood produce various Flood forms.

Carrier form
Main article: Carrier form

These bloated forms incubate Pod infectors, and release them upon death.

Spawner Forms
Artwork of the Flood spawner form. In the Halo Encyclopedia (2022 edition), this art is incorrectly used to depict the Flood infester form.
A Flood Spawner.
Main article: Flood spawner form

These forms can produce a variety of Flood forms on the move.

Flood burster
Transparent crop of the Flood burster image from the Phoenix Logs.
A Flood Burster.
Main article: Flood burster

These forms can produce a variety of Flood forms and release them once its internal pressure has reached a certain point.

Combat forms

Main article: Combat form

Once a host of any organic/sentient species has been infected, they can develop into a combat form.

Key minds

Main article: Key mind

Key minds are nodes for the Flood's collective consciousness, ranging from mobile coordinating forms to compound minds spanning entire planets.[24]

Juggernaut
Main article: Flood Juggernaut

The Flood Juggernaut is a large Flood command form directed by a seething mass of infection forms linked to multiple hosts.

Abomination
A render of the Abomination model for Halo Wars 2.
An Abomination.
Main article: Abomination

The Abomination is a mobile, low-level key mind form capable of combat that coordinates the Flood on a local scale.

Proto-Gravemind
Main article: Proto-Gravemind

A biomass of hosts, for the purpose of utilizing their minds to form a collective intelligence. A Proto-Gravemind can grow into a Gravemind given enough time and host bodies.

Gravemind
The Gravemind introduces itself to John-117.
A Gravemind.
Main article: Gravemind

This ultimate Flood form serves as the coordinating hive mind of the Flood. While they vary in size and complexity, at their height Graveminds can engulf entire worlds.

Biology

A wall covered in heavy Flood biomass: evidence of advanced infestation.
Untextured render of the Flood spore model developed by Blur for Halo 2: Anniversary's cutscenes.
A Flood spore.

The Flood is an anomaly in all known biology; their mode of growth and reproduction would make them incompatible with any natural ecosystem.[3] They are extremely adaptive, and transform hosts to suit the species' current needs.

Capable of surviving extreme environments ranging from -75 to +53 degrees Celsius and even underwater, the Flood can withstand the varied environments of all known inhabited planets. However, the Flood seem to thrive the most in moist and humid areas; it is thought that the extremely cold artificial weather patterns near certain important areas of the Halo rings are intended to hinder the Flood. [citation needed] In order to give themselves ideal living conditions, the Flood will transform the environment to better suit their needs, altering the atmosphere and covering solid surfaces with Flood biomass.

All Flood biomatter is made of a unique, undifferentiated "Super Cell", which relays information and coordinates the cellular assembly like conventional neural cells. It is also capable of flexing and moving in coordination with other cells, and can be arranged to mimic any bodily organ that is required for the Flood's use. This combination of traits has caused some to refer to it as "thinking muscle."

The Flood Super Cell physically resembles both neuron and glial cells in structure, with a central cell body and many tendril-like, branching structures.

Infection

Flood Super Cells performing cell division inside of a host.
The Pod infector; the primary method administered towards the infection of an organism.

In order to reproduce and grow in biomass, the Flood needs to infect and assimilate other life-forms. While capable of infecting nearly any organism, the Flood shows a strong preference for sentient life-forms, as assimilating their cerebral tissue allows the Flood to increase its overall intelligence.

The first stage of a Flood outbreak is typically initiated by contact with Flood spores or even more reduced forms of Flood biomatter, should no extant Flood forms be present.[58] Once a basic infestation has been established, Flood infection is typically carried out by Pod infectors, which are some of the most complex forms that can be generated from Flood biomass during the Feral Stage. Upon sighting a host, a Pod infector will leap for the most accessible area of the victim's torso; if the target is an armed human or other bipedal species, this is typically the left side of the chest below the neck, as most combatants will expose this area while shouldering their weapon.[60] Upon contact, the Pod infector will rasp away at clothing and flesh until it has burrowed into the chest cavity, mortally wounding them. As the host dies, the Pod infector extends tentacles that tap into the victim's spinal cord or other nerve center, forcing a "match" with the nerve signals previously produced by the host's living brain.

To begin the physiological conversion of a host, the Pod infector injects encapsulated Flood Super Cells into the body. As it does so, the form analyzes the entire genetic code of its host, allowing the Flood to determine the most effective use of the victim. Simultaneously, the inserted Flood cells capture and break down the host body's cells into organic raw material, which is then absorbed and assimilated into the attacking Flood cells.

The Flood infection then uses the absorbed biomass to create new masses of Flood Super Cells. As the Flood continues to use the resources of the host body, particularly its calcium reserves, the parasite's own cells build upon and augment the framework of the host to produce one of a variety of Feral-Stage Flood forms. This entire process, from the initial kill to total control over a fully mutated host body, takes only a matter of seconds.[61][62]

The infection process is not limited to living hosts; organisms that are already deceased are also susceptible to infection. As long as the host has not decomposed to the point where the body and nervous system have completely deteriorated, the Pod infector can infect and convert the host.

The Flood infection utilizes the host's biological content as the fuel for its work, which is consumed and employed at prodigious speeds. From the beginning until the end of this complex, multi-step process, the Pod infector keeps the body "alive" by chemically isolating the host's dying brain, preventing the signals of brain death from reaching the rest of the body and causing it to shut down, which is an essential part of the parasite's mission of consuming and converting the host organism.[63]

While the Pod infectors are an effective and economical means of delivering infection, particularly against armored targets, the Flood can infect hosts through other means. Flood spores, if inhaled, can transform a host into a combat form. This is the Flood's primary means of infection when an outbreak reaches a certain point: Flood spores will be deployed into a local biosphere in copious quantities, instantly overwhelming any unprotected biological targets.[64] Flood conversion may also be initiated through contact with lacerated tissue, making any Flood form a potential vector for infection.[35] Even the most basic form of Flood genetic material is virulent; once inserted into a host, it will mutate the host's DNA over the course of generations until it is capable of forming its own Flood super cells.[65] When infected by these mechanisms, host organisms tend to mutate at a more restrained level that maintains the structure and form of the infected life form, albeit with a significant number of haphazardly placed tendrils, claws, and minor organs, such as sensory stalks.[66]

Neurological

Pod infectors selectively target species that possess sentient intelligence and are of sufficient biomass, and can infest intact or lightly wounded dead bodies. As a host organism is killed by the initial attack, Pod infectors use tendrils that pierce the skin of the host and find their way to the spinal cord. They then synchronize with the host's nervous system and gain control of their body, replacing its now-absent consciousness with the ravenous and voracious psyche of the Flood. Any useful information present within the memory centers of the host's brain, such as battle strategies and technical knowledge, is retained for use by the Flood. Despite this lingering of certain memories, no trace of the original mind remains; only a simplistic and primal urge to assimilate other species drives the organism after infection. However, the entirety of the host's mental content is seemingly absorbed and added to the whole, as the Gravemind has shown to be able to simulate its victims' consciousnesses within itself.[38][67]

In isolated cases, such as when the Flood seek very specific information from an individual's mind, they utilize an alternate method of infestation that does not subdue the host's consciousness immediately despite mutating their physical form; this allows the Flood to "burrow" into the host's mind, eventually giving them access to the entire scope of the host's memories. This has only been specifically seen when the Flood sought information from Captain Jacob Keyes, such as the location of Earth, or a starship that would free them from Installation 04.[68] In addition, if the Pod infector that initiates assimilation is extremely aged or damaged somehow, the host may remain alive and conscious; Private Wallace Jenkins suffered such a fate, remaining alive and fully aware of his situation despite the mutation of his form, even exhibiting limited control over his body at times when the Flood instincts were dormant.

When the Flood have amassed enough sentient hosts to form a Gravemind, then their behavior becomes far more coordinated, and the Flood collective becomes much more deadly as a result. The Gravemind may also speak directly through combat forms and pure forms under its control.

Physiological

While baseline neurological assimilation is the same regardless of host species, the Flood infection process includes physiological transformations, which vary depending on the host and the Flood's present need for specialized combatants.

Upon being infected, higher-level species capable of combat are typically transformed into combat forms. The host's physiology is completely "rewritten" during the infection process: organ-based systems are corrupted, organ-specific functions are decentralized, and body cavities are decayed, making hitbox-selective incapacitation impossible and brute force incapacitation difficult. Due to the lack of a central nervous system, decapitation will neither stop nor slow a combat form.

Furthermore, munitions with high speed and penetration, such as sniper rifle rounds, are usually ineffective against combat forms. Picking off selective areas of a combat form leads to no wide-scale physiological impediments, and the projectile will punch through the decayed flesh rapidly, exiting through the other side of the combat form without dealing any major damage.

To facilitate melee combat, combat forms generate tentacles and claw-like structures, which protrude from the host's arms. Generally, a single arm develops into a clawed appendage, while the other retains the capacity to manipulate objects, such as firearms. The rapid growth of these structures tends to push aside and break apart the host's hands and forearms.

Combat forms can strike with devastating force, move with tremendous speed and agility, and jump to great heights, displaying physical abilities far beyond those of the host organism while it was alive.

The price for these formidable abilities is that the required metabolism is so rapid and extreme that the host organism's corpse is rapidly broken down by the parasite. This is dramatically demonstrated by the rapid decomposition and disintegration of an infected life-form, which will eventually become a stunted, bloated, mobile generator and incubator for more Pod infectors, which is commonly known as a carrier form.

Pod infectors and pure forms sense their surroundings using specialized tentacles, which are tipped with red ganglia. Exactly how these organs work is unknown; they may provide visual or chemical information, or perhaps a combination of both. In combat forms, these tentacles typically protrude from the chest cavity where the Pod infector is housed; in pure forms, they are located on the front of the "head" segment.

The described changes occur when the Flood has still yet to overwhelm any meaningful resistance in a given locale. When this has been accomplished, available organisms will no longer be used to create purpose-built combat forms but are instead used to create a Gravemind or simply broken down and processed into biomass for a Flood hive.[35][64]

Metaphysical and technological

"The Flood changes everything. Not just flesh. Space itself is infected. That's the power the Precursors once had... isn't it?"
— The Ur-Didact to the IsoDidact.[69]

Although the Flood is biological on the most basic level, as an outbreak advances, it becomes a highly powerful self-replicating swarm which will seek to harness any mechanism — philosophical, ideological or technological — to perpetuate itself and its goals. These aspects were most prominently demonstrated during the two major outbreaks in the Forerunner era, particularly the Forerunner-Flood war.[70]

The Flood's most notable meta-biological aspect is the logic plague, an abstract infection carried along information transmission and exchange, typically used to subvert AIs. Mechanisms used to perpetuate this infection range from subtle philosophical persuasion through mutual conversation to more predatory techniques, such as swarming AI networks with a self-replicating information infestation with the ability to convert most machine intelligences near-instantaneously. In the later days of the Forerunner-Flood war, the Flood was able to spread the logic plague through any of its biological forms or infected ancillas.[70]

During its war with the Forerunners, after accumulating sufficient intellectual capacity and processing power in the form of several planetary Key Minds, the Flood gained the ability to manipulate neural physics, which served as the foundation of Precursor technology.[23] This enabled it to control the Precursors' formerly dormant structures—most prominently star roads—and use them as weapons against the Forerunners, as well as manipulate space-time to become incompatible with Forerunner slipspace travel. On a more abstract level, the Flood's connection to neural physics caused the Forerunners to experience the very space-time around them as being increasingly hostile.[69] Upon the development of a Gravemind, the Flood becomes capable of what can be described as telepathy among its hosts;[3] given the Flood's origins, this ability may be related to neural physics.

The Flood is able to use any technology it has assimilated, often to a superior degree than the original creators of said technology due to its vast base of knowledge and intellect, as evidenced by its ability to transition the UNSC frigate In Amber Clad into High Charity with a precision slipspace jump normally unheard of with human ships. Similarly, it was capable of tapping into Installation 05's teleportation grid and utilizing it with great accuracy.[38]

Developmental stages

A Flood infestation goes through four distinct developmental stages:

  1. During the Feral stage, the Flood are at their simplest form: they communicate via pheromones and have the instinct to harvest enough calcium to establish a viable Gravemind.[3]
  2. The Flood become truly dangerous during the Coordinated stage: at this point, they are controlled by the Gravemind that was created in the first stage.[3]
  3. In the Interstellar stage, the Flood take control of space-faring technology which they use to consume the local star system. Subsequently, they spread throughout the galaxy to infect more hosts.[3]
  4. The Transgalactic stage is a theoretical stage in which the Flood utilizes all captured technology to depart to uninfected galaxies to further replicate.[3]

v • d • e
 
Known Flood forms
Regular Spore • Pod • Seeder • Tadpole • Dispersal pod
Growths Stalk • Blister
Other Flood Super Cell (FSC)
Infected
Structures
Hives Hive • Den • Nest • Colony • Spore Mound • Cocooned base • Blightlands
Defensive Launcher • Root
Other Porta • Vent

Countermeasures

A Marine being attacked by a Pod infector.

Small-scale

There is no confirmed way to effectively stop a Flood infection once Flood Super Cells have entered a host. The only exception is too rare and factually suspect to be considered viable: the human disease "Boren's Syndrome", a neurological condition where radiological exposure causes electrical anomalies in the host nervous system, allegedly renders the host immune to infection while living. The neural frequencies of the host are "scrambled", apparently without negative consequences to the host. However, during Flood infection, the Pod infector is unable to tap into the host's nervous system, rendering it incapable of proceeding to large-scale physiological mutation. An interesting side effect of this failed infection is that Flood DNA and cells appear to be harmlessly integrated into the host's body. This seems to provide the host with increased regenerative capabilities, although this has not been independently verified. While Boren's syndrome left untreated may render the victim immune to the Flood, leaving it untreated almost always equates to death within a couple of years. There is said to be a "one in a billion" chance that research into the disease could produce a cure.[71] However, the accuracy of this information remains highly dubious, as most of it is simply a hypothesis devised by Dr. Halsey to explain how Avery J. Johnson survived Flood infection. Johnson's "Boren's Syndrome" was actually a story invented to cover his past in the ORION Project: his symptoms were side-effects of his experimental enhancements, not a neurological disease.[72]

While the Forerunners could not find a cure for Flood infection, the Composer was designed to eliminate its effects by extracting a Flood victim's consciousness into digital form and then re-inserting it in an artificially-constructed body devoid of infection. However, the Forerunners never perfected this process and any imprinted bodies died soon after,[73] due to the Flood somehow resuming the infection in the new body. The Forerunners attempted to use various means to slow down this process, including a specially designed harness of mesh and wires, which would apparently counteract the infection. The victim would retain most or all of their mental faculties and self-control, although their body would continue to be mutated by the Flood. This countermeasure was only temporary, however, and the victim would eventually either decay and die or be fully subsumed by the Flood. Several Forerunners on Installation 07 were subjected to this countermeasure.[74][75]

Although initially used to treat those with small-scale Flood infection already present, the Composer was eventually used by the Ur-Didact to convert his Prometheans, as well as a large number of humans, into AI constructs for his war machines, in an effort to combat the Flood more effectively and without risk of infection.[16]

Large-scale

Since the Flood are a parasitic life form, they depend on other organisms for survival. Thus, the most effective way to stop a large-scale infestation is to take away their food sources. The Forerunners first attempted this by bombing large population centers and destroying inhabited star systems via supernovae, but this was seen as one of their last resorts and as such was performed too late in the war to make much difference. The last resort was the use of the Halo Array, which destroyed all sentient life in the galaxy and starved the Flood into extinction. However, the Forerunners' plan included some measures to preserve sentient life and return them to their home planets, allowing the Flood's "food source" to persist. Unfortunately, at the same time, specimens of Flood spore cells were kept for analysis on the Halo installations, allowing the threat of the Flood to be merely forestalled and not eradicated.

Environmental stimuli seem to have an effect on Flood metabolism. In the event of a Flood outbreak, Halo installations create a quarantine zone around the affected area,[76] inside which the ambient temperature is drastically lowered. At least one Flood research facility used low temperatures to keep its specimens inactive and dormant as it studied them.[77] At least one of the Halo installations' Control Rooms was located in a snowy environment, possibly to hinder escaped Flood in their attempts to reach it.

The Flood are also vulnerable to temperatures on the other end of the spectrum. plasma weapons will easily burn through Flood tissue; the Forerunners incorporated directed-energy weapons into their Sentinels, indicating that they had determined high-enthalpy energy weapons to be the best 'basic countermeasure' available to fight the Flood. Known examples of Forerunner personal weaponry appear to be geared toward the disintegration of organic matter,[78] also reflecting the Flood's vulnerability to extremely high temperatures. The UNSC has taken advantage of this weakness as well, making heavy use of flamethrowers in their engagements with the Flood.

When engaging heavily entrenched Flood structures such as colonies and Cocooned bases, orbital bombardment is usually a preferred method of elimination as direct attacks can be extremely costly.[79]

The "cure"

During the human-Forerunner wars, humanity (who at the time was a technologically advanced, spacefaring race) believed they had discovered another way to defeat the Flood. Using genetic engineering, a third of the human population was altered to carry a set of specially designed genes. This group of humans was then allowed to be infected by the Flood. Once the genes were exposed to Flood DNA, they triggered a cascade of cell destruction that spread throughout all Flood biomatter. The majority of the Flood died off as a result, although they would return in full force ten thousand years later. All information regarding this "cure" was destroyed by the humans as a final act of revenge against the Forerunners, leaving them unprepared for the second Flood strike.[80]

In reality, while humanity's efforts indeed produced the intended result and repulsed the Flood threat, it was later revealed by the Primordial that the Flood retreated by its own initiative, as humanity was not yet to be "tested" by the Flood. As such, humanity's survival was in fact due to the Flood's deliberate decision not to infect human populations, as the Precursors had determined to use the Flood to punish the Forerunners first.[18]

After the Flood began to ravage the Forerunners' ecumene, Forerunner scientists led by Master Builder Faber tried in vain to extract information on the cure from humans' ancestral memories, imprinted as part of their geas by the Librarian, unaware that the cure never really existed.

Combat behavior

The Flood emphasizes numerical superiority in engagements.

The Flood's combat strategy is simple: they throw themselves at potential hosts in huge numbers and with any and all weapons available. The parasite seeks to saturate its opponents with an excess of noise, so thoroughly it impedes the enemy's ability to process information and prevents them from organizing a meaningful resistance.[21] The Flood exhibits this behavior on all levels, from its physical swarm tactics in both ground and space to its corruption of data networks and, once an outbreak has progressed far enough, space-time itself.[69]

Flood tactics are based primarily on asymmetrical warfare, using various different forms to kill and infect enemy forces. Infection forms swarm across the battlefield in waves, reanimating any corpses and victims they can find, while combat forms attack and kill any living hosts in sight. Pure forms, if present, provide support to combat forms in a number of ways: Stalker forms move to strategic positions and mutate into either Tank or Ranged forms, which can destroy enemy vehicles or fortifications or provide suppressing fire from afar.

The Flood have a strong preference for close-range combat: combat forms possess extremely powerful melee capabilities due to their specialized claws and tentacles, and are capable of bringing a suit of MJOLNIR armor's energy shields to half strength with a single strike. Exploding carrier forms cause tremendous damage at close range, with the added benefit of spewing Pod infector in the immediate vicinity of enemies. Pod infectors are capable of attacking only at point-blank ranges, exploding in damaging bursts when meeting an opponent's personal energy shields or instantly grabbing hold of an unshielded enemy organism and infecting it.

Typical Flood combat behavior: ravenously targeting the nearest enemy, with Pod infectors running ahead of combat and carrier forms.

Combat forms can also use firearms, and have been seen clutching them in one of their hands (regardless of whether it is one-handed or two-handed). However, combat forms are very inaccurate when using ranged weaponry; it is believed that the neurological and physical effects of Flood infection significantly decrease accuracy.

The Flood collectively learn any information contained within the minds of their sentient hosts. For example, during the ill-fated raid on the Infinite Succor, after even one Sangheili was assimilated, all the Flood could open any doors and access any system on the ship immediately afterward, demonstrating the Flood's ability to share collective knowledge across their species. Furthermore, when the Flood assimilated the Prophet legate, the Minister of Etiology, the Flood reflected the high-level knowledge of the Legate through its individual organisms to Special Operations Commander Rtas 'Vadumee, mocking him with the words, "the Forerunner could not defeat us...what chance you?". The Flood, however, reflect unusual knowledge in the individual psychologies of its individual species in processes facilitating increased infestation: The Flood were sighted gathering bodies in the corners of the Covenant ship Truth and Reconciliation, perhaps making it easier for infection forms to find hosts to reanimate. The Flood were also gathering body parts for a new proto-Gravemind aboard the Infinite Succor.

Flood combat forms have been sighted crudely operating Karo'etba-pattern Ghosts, Warthogs, Wraiths, and Scorpions in ground operations, although with slow deliberation and clumsy driving and accuracy. The assimilated crewmen of the UNSC frigate In Amber Clad were also able to initiate a Slipspace micro-jump for the frigate and perform rudimentary maneuvering of the In Amber Clad, navigating the frigate from the atmosphere of Delta Halo to High Charity and then crashing the vessel into a tower, releasing Pelican dropships to rain down upon the Covenant capital.

While one of the Flood's main strengths lies in the decentralization of its compound mind and ability to be reborn from even the most carefully reduced remnant, it is nonetheless known to develop and assume a certain "core" Gravemind as an embodiment for its strategic intelligence. The Forerunners observed that the Flood exhibits a seemingly instinctive tendency to recoil and concentrate all of its otherwise distributed assets around this core when threatened on a large scale. A threat to the Flood's existence major enough to prompt such action was the imminent firing of the Halos, at which point the Flood gathered all of its available assets and launched an attack on the Ark.[81]

The Flood is very effective on a psychological level, as its parasitic nature and gruesome appearance alone are a source of horror for most of its enemies and thus serve as powerful demoralizers. It is not uncommon for survivors of Flood encounters to be severely traumatized, often to the point of delirium, after witnessing their comrades get infected or being subsequently forced to kill said infected individuals.[82][83]

Notable victims

Non-canon and dubious canon appearances

Silver Timeline

HTV-SilverTeam logo.png
 This article contains information about the Silver Timeline, and is not a part of the established Halo canon.

Kwan Ha: "What is it?"
The Mother: "It is the cohesion. The meaning. The final equilibrium. This is the end. This is the Flood."
— The Mother reveals the monster's true identity.[84]

The Mother repeatedly warns Kwan Ha of a "monster" that is approaching in visions.[85][86]

After accessing a Forerunner site on Onyx, Kwan, Dr. Halsey, and Miranda Keyes are shown a star map documenting the Flood's spread, causing Kwan to realize that the monster is real.[87]

An overeager biologist named Janine Caryyew inadvertently releases Flood spores from within a sample container collected by Kwan, Halsey and Miranda, unleashing the Flood throughout the Onyx base. The Flood quickly consumes and converts most of the base's personnel, including Admiral Margaret Parangosky. While Halsey is infected, Miranda places her mother into stasis in the hopes of finding a cure. Kwan, Soren, Kessler and Ackerson manage to escape thanks to the intervention of the Mother who warns Kwan of the true nature of the Flood, but Laera remains behind after getting infected.[84]

Gameplay

Main article: The Flood/Gameplay

Changes from Halo: Combat Evolved to Halo 2

  • In addition to making low growls, the Flood make screeches and roars, which were not present in the first game.
  • Their skin is more textured due to the improved graphics engine, giving the Flood a more realistic and necrotic look. The faces of the human forms are also more readily recognizable.
  • If a combat form has both arms shot off, it will walk slowly then explode, releasing the Pod infector that was controlling it. This eliminated the Flood Buddy glitch.
  • Rather than having to "gather bodies," the Flood can seemingly infect hosts over the course of a few minutes or even a few seconds, once again possibly due to the presence of the Gravemind, eliminating the need to gather bodies to form one.
  • Rather than corpses reanimating themselves, a Pod infector will enter the body, then reanimate it, even if the form was just eliminated.
  • If the player is attacked by a Pod infector while their shields are down, the character will die instantly. In Halo: Combat Evolved, it would simply take a bar of health away.
  • Some combat forms derived from Elites still have functional energy shields. However, they never possess active camouflage, as some in Halo: Combat Evolved's final level did.
  • Flood combat forms can use vehicles, albeit clumsily, and are also able to board the player's vehicle; they will then proceed to melee the player character and pull them from the vehicle.
  • Melee attacks do little to no damage against the Flood in Halo: Combat Evolved. They deal a bit more damage in Halo 2.
  • The Flood prefer to stay back and fire on the player rather than rush him. This could be because of the presence of the Gravemind, though, from a gameplay perspective, it is due to improved artificial intelligence.
  • In Halo: Combat Evolved a combat form would walk slowly when shooting at the player. When losing the arm that shoots, it would charge the player character. In Halo 2, combat forms either use a melee attack or shoot at the player in these situations.
  • If a player is wielding an energy sword or shotgun, a combat form may retreat. They will not do this if a player is wielding other weapons.
  • Unlike in Halo: Combat Evolved, shooting one Pod infector will not result in the rest of the swarm exploding with it. However, Pod infectors are encountered in proportionally smaller numbers in comparison to Halo: Combat Evolved.
  • Pod infectors appear noticeably less textured and detailed than in Halo: Combat Evolved.
  • Flood Combat Forms do not "feign death" anymore.
  • Splattering become more difficult as the Flood can use the vehicle momentum to hijack the vehicle.

Changes from Halo 2 to Halo 3

  • Brutes may now be infected and transformed into combat forms.
  • Melee attacks and plasma weapons were nearly useless against the Flood in Halo: Combat Evolved and Halo 2, but now can be used effectively against the Flood. The Flood are much more vulnerable to ballistic weapons, as well.
  • Pod infectors can mutate humans, unshielded Elites and Brutes into combat forms within seconds (as seen during Floodgate).
  • Pure forms are introduced.
  • Combat forms wield weapons far less frequently than in Halo: Combat Evolved and Halo 2.
  • Carrier forms release significantly more Pod infectors when they explode. The explosion does not push the player away much as in Halo: Combat Evolved and Halo 2. In addition, carrier forms are significantly more difficult to kill with projectile weapons, but they are easily destroyed by plasma weapons.
  • Running over Pod infectors while driving a vehicle can result in loss of steering and possibly overturning of the vehicle. Any Pod infectors that are hit by the vehicle will not pop.
  • Flood combat forms can no longer commandeer vehicles, like they could in Halo 2.
  • Sticking a carrier form with a Anskum-pattern plasma grenade or Spike Grenade will kill both the carrier form and the pod infectors it carries. This is also true of a supercombine explosion.
  • Any weapon wielded by humans, Elites, and Brutes will not be dropped when transforming into combat forms and the combat forms will be also unarmed.
  • Any humans, unshielded Elite, and Brutes have a low chance to resist or preventing the Pod Infector from infecting them.
  • Any Pod infector latching into surviving humans, Elite, and Brutes before the infection can be shot at to prevent infection.

Changes from Halo 3 to Halo Wars

Significant changes to the Flood's general existence were necessary due to the change of genre from first-person shooter to real-time strategy game. These are as follows.

  • Many new forms, such as bomber forms, were created for Halo Wars to expand the "roster" of Flood units. The official explanation is that these forms were created from creatures indigenous to Trove.
  • Combat forms have more tentacles than before; they resemble spikes rather than flexible appendages.
  • Grunts and Jackals can be converted to combat forms.
  • Pod infectors do not explode when they are shot in the cinematics, instead 'deflating' like punctured balloons, though they still explode in gameplay.
  • Carrier forms do not always explode upon death.
  • Hero Units that do not have shields will be instantly downed regardless of any remaining HP on them, featuring an instant kill on any enemy infantry without energy shielding.

Changes from Halo Wars to Halo Wars 2

  • The Flood now has a Super Unit, The Abomination.
  • New forms are introduced, such as Flood spawner forms, which can spawn Flood forms on the move and can spawn counter units.
  • The Flood now only infects the Squad Leader(Either Jiralhanae or Sangheili depending on the Leader) in a Grunt Infantry unit, while the rest are killed. Marine units are entirely infected, retaining Halo Wars infection traits for humans.
  • Pod infectors can now be killed more effectively with units that deal splash damage.
  • The Flood can now infest and commandeer vehicles that are still operational, except Super Units and mechanical units (Mainly Sentinels).
    • Seeder infector, a new variety of Infection Form that is able to infect air units, is introduced.
    • Flood infestor, a new kind of pure form, is introduced and specialized in infecting the vehicles.
    • Infected mechanical vehicles are immune to Isabel's Ghost In The Machine Leader Power.
  • Turret defenses have had their effectiveness reduced against Pod infectors, even those equipped with anti-infantry upgrades.
    • Mega Turret and Siege Turret were exempted from this due to providing splash damage.
  • The infection process affects all units. If the Pod Infector units doesn't match its victim (For example, Pod Infestor attacks infantry unit is not match for infection), it will be destroyed or killed. If match, the infection process can be started.
    • Any infected units will revert their health back to 100% once the infection process has been completed.
    • Infection process can be started once a unit is dying (Red Health).
  • New structures for barricades are introduced.
  • Pod infector squad size increased 3 to 4 times.
  • Biohazard sign appears every time a unit has gotten infected.

Production notes

Main-Superintendent.png Browse more images in this article's gallery page.
""He’s referring to one of the oldest Flood illustrations we have on record. It’s literally the size of a postage stamp, the full resolution original lost in the shuffle and whirl of thirteen years gone by. “That might be have been drawn in ‘97 or ‘98. We were still in the Halsted office, down in Pilsen, sort of our first real office in Chicago.” “I did some pencil sketches prior to this,” McLees notes, referring to the thumbnail sized image, “but that’s probably the first digital one. It’s actually pretty close to what the Carrier forms ended up looking like.” Well, the color tones are a bit different, I note. Instead of the muted tones that form the pockmarked surface of the slithering menace we’ve come to know, this image of the Flood Carrier features colorations more akin to healthy human skin, strokes of pink and red and ivory. “That’s because I’m color blind.”""
The earliest extant concept art of the Flood, by Robert McLees.

Miscellaneous

  • The Flood's AI routines in Halo: Combat Evolved were originally intended to be "every bit as complicated as the Covenant", but full implementation was prevented by time constraints.[88]

References

  • The name of the Flood is a reference to the biblical story of the Great Flood. Some of the parasite's history mirrors the story, in which God destroyed all life except for that which was carried by Noah's ark.
  • The Flood bear a strong visual resemblance to an enemy cut from Marathon 2, the “fungal zombie.” These were to be the result of a bioweapon invented in the final days of the ancient S'pht and reused by Robert Blake's forces on Lh'owon, designed specifically to target Pfhor. Both the fungal zombies and Flood were principally designed by Robert McLees.
  • The Flood were inspired by Christopher Rowley's The Vang.[89]
  • An alien virus named "The Flood" appeared in a 2009 special episode of the British science fiction television show Doctor Who. Although like its Halo counterpart, it is hinted to be intelligent, and is extremely virulent, transforming its victims physically, the Flood found on Mars literally uses water to transmit itself.[90] The previous episode, also a special, involved the alien planet "San Helios", a name very similar to Halo's Sanghelios.[91]

Gallery

  • Early concept art for the Flood by Robert McLees.

    Early concept art for the Flood by Robert McLees.

  • Early concept art for the Flood by Robert McLees.

    Early concept art for the Flood by Robert McLees.

  • A large unidentified Flood form seen in Origins I from Halo Legends.

    A large unidentified Flood form seen in Origins I from Halo Legends.

  • An up-close view of the same Flood form in Origins I.

    An up-close view of the same Flood form in Origins I.

  • A Spartan-IV with Flood cosmetics in Halo Infinite.

    A Spartan-IV with Flood cosmetics in Halo Infinite.

  • A Forge Flood palette on Oasis.

    A Forge Flood palette on Oasis.

  • Icon for the Flood manufacturer in Halo Infinite.

    Icon for the Flood manufacturer in Halo Infinite.

List of appearances

Sources

  1. ^ Halo: Primordium, page 53
  2. ^ a b c Halo 3, campaign level Floodgate
  3. ^ a b c d e f g h Bestiarum
  4. ^ Halo.Bungie.Org: Updated and abbreviated list, errors amended
  5. ^ Halo 2, campaign level The Heretic: "Thel 'Vadamee: "Noble Hierarchs, surely you understand that once the parasite attacked-""
  6. ^ Halo Wars, campaign level Relic Approach: "Re'gish Wamik: "What if the infection defiles the relic?""
  7. ^ Halo: Combat Evolved, campaign level Two Betrayals
  8. ^ a b c d e Halo: Silentium, page 173-175
  9. ^ Halo: Primordium, page 335-336
  10. ^ Halo Mythos, page 9
  11. ^ Halo: Silentium, page 181-182
  12. ^ a b c d e f g h Halo: Cryptum, page 268-270
  13. ^ Halo: Silentium, page 35
  14. ^ Halo: Primordium, page 226
  15. ^ Halo: Cryptum, page 131
  16. ^ a b Halo 4: Terminals
  17. ^ a b c Halo: Silentium, page 34-37
  18. ^ a b Halo: Primordium, page 364-365
  19. ^ Halo: Silentium, page 170
  20. ^ a b c Halo: Cryptum, page 272
  21. ^ a b c d e Halo 3: Terminals
  22. ^ Halo Mythos, page 32
  23. ^ a b c Halo: Silentium, page 186-187
  24. ^ a b Halo Waypoint: "Canon Fodder - Deluge Delights"
  25. ^ Halo: Cryptum, page 293-294
  26. ^ Halo: Silentium, page 326
  27. ^ Halo 5: Guardians, REQ card: Temple Wraith description
  28. ^ Halo Encyclopedia (2022 edition), page 224
  29. ^ Halo Wars
  30. ^ Halo: Tales from Slipspace: Something Has Happened
  31. ^ a b Halo: Combat Evolved
  32. ^ Halo Graphic Novel: The Last Voyage of the Infinite Succor
  33. ^ a b c Halo: The Flood
  34. ^ Halo: Combat Evolved, campaign level Keyes
  35. ^ a b c Halo: Evolutions: The Mona Lisa
  36. ^ Halo 2, campaign level The Oracle
  37. ^ Halo 2
  38. ^ a b c d Halo 2, campaign level Gravemind
  39. ^ Halo 2, campaign level High Charity
  40. ^ Halo 2, campaign level The Great Journey
  41. ^ Halo 3, campaign level The Storm
  42. ^ Halo 3, campaign level The Covenant
  43. ^ Halo 3, campaign level Cortana
  44. ^ Halo 3, campaign level Halo
  45. ^ a b c Halo: Official Spartan Field Manual, PART 09: ALIEN FACTIONS, The Flood (p. 172-175)
  46. ^ Halo: Spartan Assault: Flood missions
  47. ^ Halo 4: The Essential Visual Guide, page 200
  48. ^ Halo 5: Guardians: Flood visor description
  49. ^ Halo Wars 2: Phoenix Logs - Idle Hands II
  50. ^ a b c d Halo Wars 2: Awakening the Nightmare, campaign level Manifestation
  51. ^ Halo Wars 2: Awakening the Nightmare, campaign level What Could Go Wrong?
  52. ^ Halo Wars 2: Awakening the Nightmare, campaign level Fighting Retreat
  53. ^ Halo Wars 2: Awakening the NightmareLight the Fuse, campaign level {{{Level}}}
  54. ^ Halo Wars 2: Awakening the Nightmare, campaign level The Archive
  55. ^ Halo Wars 2: Phoenix Logs - Proto-Gravemind
  56. ^ Halo: Shadows of Reach: Sacrifice
  57. ^ a b Halo Wars 2: Phoenix Log - Pod Infectors
  58. ^ a b Halo Waypoint: Flood
  59. ^ Bungie.net: One Final Effort (4/16/2010)
  60. ^ Halo: The Art of Building Worlds
  61. ^ The Art of Halo, page 28
  62. ^ The Art of Halo, page 50
  63. ^ The Art of Halo, page 48
  64. ^ a b Halo: Silentium, page 231-232
  65. ^ Halo: Cryptum, page 268-270
  66. ^ Halo Graphic Novel, page 19-54
  67. ^ Halo: Silentium, page 251
  68. ^ Halo: The Flood, page 175
  69. ^ a b c Halo: Silentium, page 223-224
  70. ^ a b Halo: Silentium, page 214-215
  71. ^ Halo: First Strike, page 244-246 (2001 edition)
  72. ^ Halo Graphic Novel: page 122
  73. ^ Halo: Silentium, page 40
  74. ^ Halo: Primordium, page 268
  75. ^ Halo: Primordium, page 322-323
  76. ^ Halo 2, campaign level Quarantine Zone
  77. ^ Halo 3: multiplayer level Cold Storage
  78. ^ Halo 4
  79. ^ Halo Wars 2: Phoenix Logs - Cocooned Base
  80. ^ Halo: Cryptum, page 267-272
  81. ^ Halo 3: Terminal 3
  82. ^ Halo: Combat Evolved, campaign level 343 Guilty Spark: Paranoid Marine
  83. ^ Halo 3, campaign level Floodgate: Suicidal Marine
  84. ^ a b Halo: The Television Series, episode Halo
  85. ^ Halo: The Television Series, episode Aleria
  86. ^ Halo: The Television Series, episode Onyx
  87. ^ Halo: The Television Series, episode Thermopylae
  88. ^ EDGE magazine - September 2003, page 109
  89. ^ Gamespot: Halo - Q&A
  90. ^ Doctor Who, The Waters of Mars
  91. ^ Doctor Who, Planet of the Dead

v • d • e
 
Known Flood forms
Regular Spore • Pod • Seeder • Tadpole • Dispersal pod
Growths Stalk • Blister
Other Flood Super Cell (FSC)
Infected
Structures
Hives Hive • Den • Nest • Colony • Spore Mound • Cocooned base • Blightlands
Defensive Launcher • Root
Other Porta • Vent

v • d • e
 
Polities, organizations, and civilizations
Contemporary
Interstellar/Interplanetary organizations Banished • Covenant • Swords of Sanghelios • Created • Children of Oth Sonin • Concert of Worlds • Frieden • Insurrectionists (Arcadia/Harvest • New Colonial Alliance • People's Occupation • People's Occupation Government • Secessionist Union (Eridanus secessionists) • United Rebel Front) Koslovics • San'Shyuum flotilla (Dhas Bhasvod's Covenant • Tem'Bhetek's faction) • Unified Earth Government (Colonial Authority • United Nations Space Command)
Planetary/Local organizations Castoffs • Defenders of the Sanctum • Eridanus Government • Gao Republic • Insurrectionists (Bandusa • Freedom and Liberation Party • Galodew Emancipation • Gao Liberation Force • Sundered Legion) • Joyous Exultation Covenant • Khabnathwa • Neo-Frieden • Prospector Party • Sedran government • Stewards • United Nations (Nations) • Unggoy government • Ussans • Venezian Militia
Private organizations 'Gajat mercenaries • Shields of Requiem
Religious organizations Covenant remnants ('Mdama's Covenant • 'Nyon's Covenant • Keepers of the One Freedom • Vol's Covenant) • 'Refumee's heretics • Servants of the Abiding Truth
Ancient
Interstellar organizations Ecumene • Human-San'Shyuum alliance • Ancestors
Planetary/Local organizations Council of City States • Erde-Tyrene civilization • Sedaaro • Reformists (Keepers of the Dawn) • Stoics • Tudejsa
Protogenic
Italics denote entities that are confirmed or assumed to be defunct given currently available information.
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