Louis Pasteur began as a chemist, investigating the different optical properties of substances with apparently similar chemical composition. His work on crystals helped establish that molecular arrangement could matter to physical behaviour. Posts in Strasbourg and Lille led him toward industrial problems. Studies of fermentation showed that specific kinds of microorganisms were associated with particular processes, challenging accounts that treated fermentation as a purely chemical breakdown.

During the early 1860s Pasteur's experiments with carefully handled nutrient broths challenged spontaneous generation. His swan-neck flasks admitted air while limiting contamination by particles carrying microorganisms. The work strengthened the case that microbial growth arose from existing organisms under the conditions tested. He developed heating methods to reduce spoilage in wine and beer, giving rise to pasteurization. Investigations of silkworm disease combined observation with methods for identifying and separating infected stock, assisting an industry under severe pressure.

Pasteur then pursued infectious disease and vaccination with a team of collaborators. Work on chicken cholera and anthrax explored ways of reducing an organism's capacity to cause disease while retaining its ability to stimulate protection. A public anthrax vaccination demonstration at Pouilly-le-Fort in 1881 helped establish the programme's reputation. Priority disputes and differences between his public claims and procedures complicate this history. His research formed part of a wider development of germ theory involving other scientists and physicians.

In 1885 his team administered a series of rabies inoculations to Joseph Meister after a dog attack. Pasteur was not a physician, and the experimental treatment raised substantial ethical and medical questions. Meister survived, and further treatments attracted international support. Public fundraising enabled the Pasteur Institute to open in Paris in 1888, providing a research and treatment centre. Despite the effects of a stroke and declining health, Pasteur continued to direct work until his final years; he died in 1895. His career helped join chemistry, microbiology and medicine, while the institute gave that programme an enduring organizational form. Its achievements rested on a collective scientific enterprise as well as his leadership.