Athens, Ga. – Bacteria don't have easy lives. In addition to mammalian immune systems that besiege the bugs, they have natural enemies called bacteriophages, viruses that kill half the bacteria on Earth every two days.
Still, bacteria and another class of microorganisms called archaea (first discovered in extreme environments such as deep-sea volcanic vents) manage just fine, thank you, in part because they have a built-in defense system that helps protect them from many viruses and other invaders.