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

Pasteur and Koch's germ theory of disease in the 1860s–1880s proved that microorganisms cause infection, replacing centuries of miasma and humor theories. Pasteur's experiments demonstrated fermentation and infection causation, while Koch's postulates—still the gold standard—identified specific bacteria for tuberculosis, cholera, and anthrax. This single intellectual foundation transformed surgery via antiseptic techniques, birthed vaccines and antibiotics, and drove life expectancy from about 40 years in 1850 Europe to over 80 today—a 100% increase. Outperforms #3 in direct human impact: germ theory saved more lives than any other scientific discovery, with antibiotics alone adding 10–20 years to average lifespan across the 20th century.