The Flood (sometimes called The Parasite by the Covenant) is a virulent, parasitic lifeform that thrives by consuming other sentient life of sufficient biomass.

The Flood appears in three primary forms: Infection, Combat, and Carrier, but also can evolve into the 'Gravemind' form that appears to act as a kind of hive-mind for the entire species. All of these forms have an attraction to any calcium-based life form of sufficient mass to accommodate them and will try to attack/infect any they encounter.

NOTE: Calcium? Perhaps they meant "carbon".