Flood

“One single Flood spore can destroy an entire species.” --Rtas 'Vadum, Halo 3.

The Flood(meaning 'the dead reincarnated' in Latin) or The Parasite as they are known to the  Covenant are highly virulent 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, such as Sangheili, and Jiralhanae, 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. 