Germ Theory (1860s)

Louis Pasteur's experiments disproving spontaneous generation and demonstrating that specific microorganisms cause specific diseases in the 1860s, confirmed and extended by Robert Koch's work identifying the bacteria causing tuberculosis, anthrax, and cholera, gave medicine its first causal understanding of infectious disease. Before germ theory, illness was explained by miasma ("bad air"). After it, rational treatments, vaccines, sanitation, and antibiotics became possible.