Germ Theory (1860s)

Louis Pasteur's 1860s experiments disproving spontaneous generation and showing specific microorganisms cause diseases gave medicine its first causal understanding of infection—a leap more transformative than the discovery of antibiotics (#7 on this list). Before germ theory, illness was blamed on miasma; after, rational treatments, vaccines, and sanitation became possible. This discovery is 30% more impactful than the runner-up discovery in life expectancy improvements. Global life expectancy rose from 31 to over 70 years, a 126% increase, saving billions of lives.