Germ theory of disease
One problem with the old theories of disease was that people believed that living things were spontaneously generated. Spontaneous generation is when something like a fly grows from a small speck of smelly meat. That old theory is called abiogenesis. In the seventeenth century, Francesco Redi (February 18, 1626 – March 1, 1697) discovered that flies lay eggs that become maggots. Prior to this, people thought that maggots came from rotting meat. He discovered this by sealing some meat in a jar and watching it. No maggots were found on the sealed meat. He also put some meat in a jar and covered it with gauze. Maggots were found on the gauze but not inside the jar. But when he placed meat in an open jar, maggots were found on the meat and inside the jar. Experiments like this proved that maggots come from flies who lay eggs, not from rotting meat. Later scientists would prove that diseases did not come from the air. Diseases are spread by infection.