PHAGE ARMS RACE. “In the prokaryote world, competition for environmental resources is not restricted to competition between different bacterial species. Infectious agents, such as plasmids and bacteriophage viruses, also compete with bacteria: in this case, it is the bacteria itself that is the resource, and the evolutionary arms race involves the spreading of the heterogeneous mobile genetic elements in the bacterial colony.”
http://www.nature.com/cdd/journal/v9/n4/full/4400950a.html