Germ theory of disease
One problem with the old theories of disease was that people believed that things spontaneously generated. Spontaneous generation is when something like a fly grows from a small speck of smelly meat. That old theory is called abiogenesis. 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.