The lost and the damned.
Flood-infected sentients serve the parasite as tools and weapons while simpler creatures are twisted into foul abominations that seek only to devour and fuel the parasite's growth. There is no cure or treatment that can save these victims, and even death may not save their consciousness from being absorbed into the Flood collective for later use.
Though their bodies are bent to the Flood's will, occasionally the victim can still whisper for mercy or cry out in pain from ruined lips and half-consumed throats. Or worse, the parasite will speak on their behalf, using stolen thoughts and memories as a psychological weapon.
Most infantry and vehicles with a living crew who take lethal damage from Flood are converted into Infected units. Exceptions include non-organic units (Sentinels), Huragok living machines (Engineers), the enigmatic Lekgolo (Hunters), and Super units (who self-destruct when compromised). Heroes also ensure they are not taken by the Flood.
Infected vehicles are hideously warped versions of their former selves, their crew having been converted into spore-packed pustules and mucus-like growths which hold the damaged vehicle together. Infected vehicles are weaker than their uninfected counterparts, but when they are destroyed infection forms have a chance of bursting from the wreckage.
Infected infantry mutate into combat forms that retain the weaponry they had before conversion. Infected infantry gain a powerful tentacle-scythe melee attack and the ability to leap short distances.