French chemist turned microbiologist, Pasteur showed that fermentation, spoilage, and disease are driven by living microorganisms. His swan-neck-flask experiments helped disprove spontaneous generation, and he established pasteurization — gentle heat that kills spoilage microbes — still applied to milk worldwide. Working alongside Robert Koch and often in rivalry with him, he helped cement the germ theory of disease, then developed vaccines for chicken cholera, anthrax, and rabies, the last tested publicly when nine-year-old Joseph Meister survived a bite in 1885. Gerald Geison's 1995 study of his laboratory notebooks revealed gaps between the public and private Pasteur.
Louis Pasteur believed that the things which make us sick are alive, too small to see, and everywhere. In a Paris laboratory he proved it. He showed that wine soured and meat rotted because microscopic creatures fed on them, and that gentle heat could kill those creatures before they did their work. He boiled broth in flasks with long curved necks, left them open to the air, and watched them stay clear for months, because the dust carrying the germs could not climb the bend. From that he argued that the microbes appearing in broth came from other microbes carried by dust, not from the broth itself. Then he turned his microbes into weapons against themselves, weakening them until they could train the body to resist. In 1885 he gambled on a nine-year-old boy bitten by a rabid dog, injecting a treatment barely tested in animals. The boy lived. The legend that grew around Pasteur was tidier than the man, but the lives he saved were real.