The Flood (Latin is Inferi redivivus meaning "The dead reincarnated" or The Parasite, as they are called by the Covenant), are a species of parasitic organisms that can reproduce and grow by consuming sentient life forms of sufficient biomass and cognitive capability. The Flood was responsible for consuming most of the sentient life in the galaxy, notably the Forerunners, during the 300-year-long Forerunner-Flood War. The Flood presents the most variable faction in the trilogy, as it can infect and mutate Humans and Covenant species into Combat Forms. They are widely considered to be the greatest threat to the whole existence of life, or, more accurately, biodiversity, in the Milky Way galaxy.
As the Forerunner-Precursor war drew to a close, the few remaining Precursors that were pushed out beyond the edges of the galaxy to Path Kethona sought vengeance against the Forerunners. A few Precursors were spared by the Forerunners, while others became a dust which could regenerate into their past forms. Over time this dust became so corrupted that it could only cause horrific diseases. The Precursors saw a chance for revenge and so they created the Flood to destroy the Forerunners as a final act of vengeance against them.