Fonts of malign intelligence.
After an infestation is established the parasite transitions from feral aggression to coordinated conversion by creating specialised Flood organisms that network their individual intelligences. These key minds act as coordinators for nearby Flood, leveraging wisdom harvested from the parasite's hosts to expand, outwit, overwhelm, and consume other life.
As the Flood grow in number these key minds combine and expand into larger, exponentially more intelligent networks. The appearance of these vile collectives typically indicate most local animal life and sentient hosts have been infected or consumed. Once a critical mass of key minds have combined the Flood's centralized intelligence can begin to emerge in their seething mass of corrupted flesh: a Proto-Gravemind that sifts through and feeds on the memories of every Flood-infected victim.