Flood

From ILB

The Flood are an intrusive viral life-form that the Forerunners studied on Halo. They devour other organisms, consume their knowledge and find ways to devour other organisms.

More on The Flood via Halo Library (http://library.psyjnir.net/?id=4) and The Halo Story Page (http://halosm.bungie.org/story/flood.html).