Flood
From Halopedia, the Halo wiki
Summary
The Flood are a group of parasitic life forms that work to consume everything with sufficient biomass in the galaxy. In natural form, The Flood are reletively harmless. However, after attaching themselves to a host body, they decay and mutate the body to be a soldier or to perpetuate the Flood species by incubating Flood eggs until they've fully developed.
The Flood were kept by The Forerunners for scientific study on the Halos.