Feral Stage

The Feral Stage is the first phase of a Flood outbreak. In this state, the Flood operates in a very stable manner and their existence forms around any adjacent sentient life forms – spreading from one group to another. Through its endless waves of Infection Forms, the Flood breach the security placed by Forerunners and attack all sentient life. Victims become hosts to the Infections Forms and are mutated into Combat Forms. During this period of time, the Flood attacks with no strategy, becomes extremely aggressive and berserk.

As was witnessed during the events of Halo: Combat Evolved and depicted in The Last Voyage of the Infinite Succor, 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 parasites and uninfected sentient beings, allowing a series of widespread chemical reactions to occur across the reserves of calcium and flesh they’ve collected, eventually creating what is known as a Proto-Gravemind. Once a Proto-Gravemind has consumed enough corpses and becomes a full grown Gravemind, the infection can proceed into the Coordinated Stage.

Trivia

 * This is the stage that the Flood most likely experienced during Halo: Combat Evolved