In the Halo universe, the parasitic species that consumes intelligent life is the Flood.
The Flood is a collective parasitic organism capable of infecting sentient hosts and converting their bodies into combat forms. It can also absorb biological knowledge, allowing more advanced Flood forms to coordinate and develop intelligence.
The species is central to the history of the Forerunners. The Forerunners built the Halo installations as a last-resort weapon because the rings could destroy the sentient life the Flood needed to survive. This prevented the parasite from spreading, but it also ended the Forerunner civilization’s ability to continue normally.
The Flood is not the same as the Covenant or the Banished. Those are organized factions with military and political aims; the Flood is a biological threat that can infect members of several species. Its appearance in Halo: Combat Evolved changes the game from a conventional alien war into a survival crisis.