Pasteur’s studies on microorganisms inspired him to pursue the study of infectious diseases. While studying an epidemic in silkworms that was disrupting France’s silk industry, he isolated the microorganisms causing the disease. This finding led him to propose the germ theory, which simplistically states that many diseases are caused by microorganisms too small to see without magnification. The germ theory would revolutionize the medical world and have a number of important practical consequences, including increased hygiene standards in the medical community and a newfound interest in disease-causing bacteria in the research community.

By the early 1870s, Pasteur had already established himself as a renowned leader in research, and in 1877 Pasteur began to fully immerse himself in the study of disease. At the time, Pasteur was studying chicken cholera (Pasteurella multocida), a diarrhoeal disease that was destroying the breeding chicken population. Influenced by Edward Jenner, Pasteur reasoned that if a vaccine could be found for smallpox, vaccines could be found for all diseases.

By 1878, Pasteur had succeeded in culturing the causative virulent bacteria of chicken cholera and began inoculating chickens. However, many chickens died after the procedure so Pasteur continued to study the disease, looking for safer inoculation methods. It was during this study that Pasteur changed the field of virology forever.

In 1879, Pasteur observed, by chance, that old bacterial cultures lost their virulence. He had instructed an assistant to inject the chickens with a fresh culture of the viral bacteria before a holiday. The assistant, however, forgot and went on vacation. When he returned a month later, he performed the procedure using the old cultures. Unexpectedly, the chickens only showed mild signs of the disease and survived. When they were healthy again, Pasteur, intrigued by the results, injected them with fresh bacteria. The chickens did not become ill. Pasteur reasoned the factor that made the bacteria less deadly was exposure to oxygen.