During the mid- to late 19th century Pasteur demonstrated that microorganisms cause disease and discovered how to make vaccines from weakened, or attenuated, microbes. He developed the earliest vaccines against fowl cholera, anthrax, and rabies.
At the same time Pasteur began his fermentation studies, he adopted a related view on the cause of diseases. He and a minority of other scientists believed that diseases arose from the activities of microorganisms—germ theory. Opponents believed that diseases, particularly major killer diseases, arose in the first instance from a weakness or imbalance in the internal state and quality of the afflicted individual. In an early foray into the causes of particular diseases, in the 1860s, Pasteur was able to determine the cause of the devastating blight that had befallen the silkworms that were the basis for France’s then-important silk industry. Surprisingly, he found that the guilty parties were two microorganisms rather than one.
