Phage: Virii that use bacteria as hosts. Phages often kill their host immediately, these are often used as antibacterial agents. Others cause a permanent modification of the hosts DNA by the implantation/transmutation of RNA without causing death.

Phage and bacteria are in a constant evolutionary struggle for supremecy.

For much much much more information, see Greg Bear's newest book: Darwin's Radio.

Not to be confused with Star Trek's The Phage, an incredibly disfiguring disease (I'm still trying to work out the correlation).