CONCEPT
Germ Theory of Disease
The claim that specific microorganisms cause specific diseases — developed by Pasteur from the 1860s through the 1880s as the organizing framework of modern medicine.
Germ theory holds that infectious diseases are caused by specific microorganisms — distinct pathogens, each with characteristic organisms, transmission pathways, and pathological effects. Pasteur developed the theory through a sequence of investigations: fermentation as biological activity (1856–1860), spontaneous generation disproved (1859–1864), silkworm diseases identified and controlled (1865–1870), anthrax bacillus characterized (1876–1881), chicken cholera attenuation discovered (1879–1880), rabies vaccine developed (1884–1885). Each investigation built on the previous, and each required the accumulated strata of Pasteur's geological formation. The theory's acceptance transformed nineteenth-century medicine from a discipline organized around symptoms and humors into one organized around causative agents. It is the foundation of modern microbiology, epidemiology, antisepsis, sterilization, vaccination, and antibiotic therapy.
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The theory was not original to Pasteur — Girolamo Fracastoro in the sixteenth century, Agostino Bassi in the 1830s, and Ignaz Semmelweis in the 1840s had proposed or demonstrated microbial causation for specific conditions. Pasteur's contribution was the systematic experimental demonstration across multiple pathogens and the development of interventions