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
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).