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Miasma theory

The miasma theory was a prominent idea in the 18th and 19th centuries that gave a false explanation of how deadly epidemics like cholera, yellow fever and malaria ("bad air") originated and spread. It said that illnesses were caused by breathing in a mysterious "miasma" --a harmful vapor that arose from decaying organic matter. Epidemics often came in the summer because that is when people spent more time outside. The theory motivated an enormous emphasis on public sanitation in major cities to remove smelly pollution, especially human and animal excrement, from streets and back alleys. The theory collapsed when physicians accepted the new germ theory of disease in the late 19th century. Germs coughed up by an infected person or spread by certain types of mosquitos or hookworms were the real reason people caught an infectious disease.