Pasteur Develops Germ Theory

In 1864, the French chemist Louis Pasteur publicly demonstrated that microorganisms are responsible for fermentation and spoilage, a result that became one of the pillars of germ theory. The idea that specific living microbes cause disease would reshape medicine, surgery and public health over the following decades. It is worth noting that "germ theory" was not the work of a single person or a single year. Pasteur's experiments of the 1860s were a crucial step in a longer story, and the link between microbes and human illness was established gradually, with major contributions from others.

For centuries, most physicians explained disease through ideas such as miasma, the belief that illness arose from foul air, or through imbalances within the body. Many scientists also accepted spontaneous generation, the notion that living organisms could arise from nonliving matter. Earlier figures had pointed in a different direction. Antonie van Leeuwenhoek had observed microscopic life in the seventeenth century, and Ignaz Semmelweis and John Snow had linked hand hygiene and contaminated water to disease in the 1840s and 1850s, though without a widely accepted explanation.

Pasteur came to the problem through chemistry and the practical concerns of French industry. Studying fermentation in wine, beer and vinegar, he argued that specific microorganisms drove each process, and that unwanted microbes caused souring. His well-known swan-neck flask experiments, which he presented to the public in 1864, showed that broth stayed sterile when airborne particles were kept out, but became cloudy with microbial growth when exposed. This undercut spontaneous generation and showed that microbes came from other microbes. His work also led to pasteurization, the gentle heating of liquids to reduce spoilage.

The consequences extended well beyond the wine trade. The surgeon Joseph Lister, aware of Pasteur's findings, began using carbolic acid in the mid-1860s to prevent infection in wounds, helping to launch antiseptic surgery. In the 1870s and 1880s, Robert Koch in Germany developed methods to isolate bacteria and linked particular organisms to diseases such as anthrax and tuberculosis. Pasteur himself later developed vaccines, including those for anthrax and rabies, applying the principle that weakened microbes could provoke protection.

Germ theory underpins much of modern medicine, from sterilization and sanitation to vaccines and, in the twentieth century, antibiotics. Historians often caution against crediting Pasteur alone, and acceptance of the theory was uneven, with some physicians resisting it for years. Still, his careful experiments gave the idea persuasive public evidence, and his name remains closely tied to the shift toward a scientific understanding of infectious disease.