Viruses also come in many different forms. They are unimaginably tiny, far smaller than bacteria. Ten thousand of them lined end to end can fit between the letters in the words of this sentence.

Unlike bacteria, protozoa and fungi, viruses cannot breathe, eat, move or expel wastes. They consist of a tiny bundle of genes carried in a shell.  Viruses are strange things that straddle the fence between the living and non-living. If they're floating around in the air or sitting on a doorknob, they're about as alive as a rock. But if they come into contact with a suitable plant, animal or bacterial cell, they spring into action. They infect and take over the cell like pirates hijacking a ship.

The viruses only purpose is to reproduce. The virus takes over the reproductive machinery of the infected cell and uses it to multiply itself. Some viruses do not greatly affect the infected bacteria, plant or animal. But the viruses that cause diseases like polio, hepatitis or AIDS can be deadly.