Germ Theory of Disease (Pasteur and Koch, 1860s–1880s)

Before Louis Pasteur and Robert Koch, disease was attributed to "bad air" (miasma), imbalanced humors, or divine punishment. Pasteur's experiments in the 1860s proved that microorganisms cause fermentation and infection, while Koch's work in the 1870s–1880s identified the specific bacteria responsible for tuberculosis, cholera, and anthrax, formalizing Koch's Postulates as the evidentiary standard for proving causation. Germ theory transformed surgery (Lister's antiseptic technique), gave birth to vaccines, antibiotics, and public sanitation, and is the single intellectual foundation of all modern medicine. Before 1850, average life expectancy in Europe was around 40 years; germ theory's consequences helped push it past 80.