Louis Pasteur formulates the germ theory of disease: microorganisms are the direct cause of infectious diseases. His presentation to the Académie de Médecine in 1878 consolidates decades of experimental work — fermentation, pasteurization, refutation of spontaneous generation — into a unified causal framework. Robert Koch simultaneously develops Koch's Postulates (1884) as a methodological criterion for establishing microbial causality. Both bodies of work are independent and complementary.