From Halopedia, the Halo wiki
The Feral Stage is the first phase of a Flood outbreak. In this state, the Flood operates in a very aggressive manner, and their expansion depends on finding sentient life forms and expanding their overall biomass. Victims become hosts to the infection forms, and are mutated into combat forms. During this period of time, the Flood attacks with little to no strategy, seeming to focus primarily on consumption of intelligent beings and biomass and thus expanding and strengthening their own forces.
The Feral Stage is typically initiated either by an outbreak of extant infection forms, or the infection of various sentient and non-sentient species by Flood spores and other microbiological vectors. This infection soon becomes parasitic and causes the growth of Flood biomass in the hosts as well the generation of new infection forms to further the infestation. At some point during the Feral Stage, the Flood begin to collectively shift their objective. When this happens, Combat Forms will work together to accumulate the biomass of both infected victims and uninfected sentient beings, allowing a series of widespread chemical reactions to occur across the reserves of calcium and flesh they have collected, eventually creating what is known as a Proto-Gravemind. Once a Proto-Gravemind has absorbed enough biomass and becomes a full grown Gravemind, the infection can then proceed into the Coordinated Stage.
The Flood at this Stage does possess basic intelligence, for example on the level 343 Guilty Spark the Flood deliberately leaves some bodies uninfected to lure others into an ambush. This befalls the Marine squad which investigates dead Elites and John-117 who investigates the dead Marines.
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