Flood (Halo)

The Flood is a fictional alien parasitic life form in the Halo video game series. They are introduced in Halo: Combat Evolved as a secondary enemy to the game's protagonist, Master Chief, and return in Halo 2. They are depicted as highly virulent and deadly, and with the ability to quickly incapacitate and infect their victims. The Flood mutate their hosts into several different forms to serve different purposes. All of these forms have an attraction to any lifeform of sufficient mass to accommodate them and will try to attack or infect any potential host they encounter.

The Flood make their first appearance in Halo: Combat Evolved's sixth level, "343 Guilty Spark." After reaching Halo's control room in the previous mission, Master Chief is sent by the AI Cortana to try and stop Captain Jacob Keyes and his UNSC Marines from reaching a weapons cache they believed was hidden in a Forerunner structure by a swamp. In reality, the cache is false intelligence; instead of finding weapons, Keyes finds that a sizable population of Flood has been released from stasis by the Covenant. The Flood quickly escape containment and spread across the ringworld, threatening not only Humanity and the Covenant, but also the entire galaxy if they escape.