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).