How did Louis Pasteur connect microorganisms with fermentation and disease?

Louis Pasteur fundamentally transformed science by demonstrating that microorganisms are the active agents behind both fermentation and infectious disease.12

Fermentation

In the mid-1850s, while studying alcoholic fermentation, Pasteur provided experimental evidence that fermentation is not a spontaneous chemical process but is caused by specific living microorganisms. He showed that each type of fermentation—such as the production of lactic acid in milk or the conversion of sugars in beer—is associated with a particular organism. This discovery led to the germ theory of fermentation and allowed him to develop pasteurization, a process of heating liquids to kill spoilage-causing microbes.324

Disease

Pasteur extended these findings to medicine, proposing that if microbes cause fermentation, they could also cause disease. By disproving spontaneous generation through his "swan-neck" flask experiments, he established that microbes must come from the environment. His work on silkworm diseases and his confirmation that the anthrax bacillus causes infection provided definitive proof for the germ theory of disease. This insight enabled him to pioneer vaccination by using attenuated or neutralized agents to induce immunity, ultimately launching the era of preventive medicine.256