In 1854, as a cholera epidemic killed hundreds in London, an English physician named John Snow was determined to find out how the disease was transmitted.

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Snow doubted the prevailing belief that disease was spread by breathing “bad air.” He noted that the disease was centered near a public water pump on Broad Street. The water in that area, he argued, was polluted by sewage, while people in other neighborhoods who drank from unpolluted wells, were unaffected.

When the handle was removed from the suspect pump, the epidemic ebbed. And Snow had demonstrated that cholera is spread not by air

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