If you have ever suffered from a cold or needed an inoculation in order to go to school, then you know that everyday life can expose you to contagious disease. Many major health problems are caused by tiny microorganisms such as bacteria and viruses that are impossible to see without a powerful microscope. Only in the last two hundred years did scientists propose the idea that diseases are caused and transmitted by microorganisms. The connection between microorganisms and disease is generally called the germ theory.

Since the time of the ancient Greeks, people had believed in the theory of spontaneous generation, the idea that living things can arise from nonliving substances like flies coming from rotting meat. Even a rational thinker like Theophrastus (372-287 B.C.), who is considered the father of botany, did not know that living things can come only from other living things...