Louis Pasteur finally disproved spontaneous generation through an experiment where beef broth was sterilized through boiling in two flasks, one that was exposed to air and another that was protected from it. The one that was exposed to contaminants clouded, showing microbial growth, while the sealed one did not.
Louis Pasteur is famous for his contributions to the understanding of disease and microbes in general. The process of pasteurization, where milk, wine and other consumables are treated to stop bacterial contamination, was both discovered by and named for him. Pasteur was the first to show that fermentation is caused by micro-organisms and that oxygen is not required for this process.
Pasteur did not originate germ theory, but he was the one who provided sufficient experimental evidence to convince most of the scientific community that it was true. He was also the first to state the principles of vaccination, and he created the first vaccines for rabies and anthrax. He discovered these principles after a mishap while studying chicken cholera, normally a fatal disease. He treated the chickens with a bacterial culture that had spoiled, which produced only mild symptoms. Afterward, he could not re-infect the chickens, even with viable bacteria.