Until the middle of the nineteenth century most scientists accepted that living organisms, at least the simplest ones, could arise spontaneously from non-living matter, a maggot from rotting meat, a microbe from broth left standing in the open air. In 1859 the French Academy of Sciences offered a prize for whoever could settle the question experimentally, and Louis Pasteur set out to win it, designing what became one of the most elegant experiments in the history of science.
Pasteur boiled nutrient broth in flasks with long, curved swan necks that let air reach the liquid while trapping dust and microorganisms in the bend of the glass. Broth prepared this way stayed sterile indefinitely, but the moment Pasteur tilted a flask so the broth touched the trapped dust in the neck, microorganisms grew. The experiment showed that air itself was not the source of life in the broth, dust-borne microbes were, and by 1864 it had effectively ended serious scientific belief in spontaneous generation.
Pasteur went on to argue that the same invisible microorganisms responsible for fermentation and spoilage were also responsible for disease, the germ theory of disease that transformed medicine. His applied work followed directly: the heat treatment now called pasteurization, developed first to stop wine and beer from spoiling, and vaccines against anthrax, demonstrated publicly at Pouilly-le-Fort in 1881, and rabies, first used on a human patient, nine year old Joseph Meister, in 1885. Meister survived a series of otherwise fatal bites from a rabid dog, and the treatment's success made Pasteur an international figure whose name still identifies the institute in Paris founded to continue his work.