Feral Stage

The Feral Stage is the first phase of a Flood outbreak. In this stage, Infection Forms 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 berserks. Once a Proto-Gravemind has consumed enough corpses and becomes a full grown Gravemind, the infection can procede into the Coordinated Stage.

Trivia
this is the stage that happened during Halo CE