Feral Stage

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. 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 thus expanding their own forces.

As was witnessed during the events of Halo: Combat Evolved and depicted in the Halo Graphic Novel, 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 have 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 then proceed into the Coordinated Stage.

Trivia

 * This stage was the first to be encountered by the Forerunner, although that fated meeting quickly led to the Flood expanding into its Coordinated Stage.
 * This is most likely the stage the Flood were in when encountered in Halo: Combat Evolved and Halo Wars.