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How did John Snow connect a London cholera outbreak to contaminated water in 1854?

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A woman in a pink dress receives medical treatment from nurses administering an IV at a crowded hospital in Saint-Marc, Haiti. — cholera A woman with cholera being treated by nurses at St. Nicholas Hospital in Saint-Marc, Haiti, 2010.

In 1854, English physician John Snow challenged the prevailing miasma theory—which attributed disease to foul-smelling air—by demonstrating that cholera was a waterborne illness.12

He connected the outbreak to contaminated water through two primary investigations:

The Broad Street Pump: Snow mapped cholera deaths in London’s Soho district and observed that cases clustered around a specific public water pump on Broad Street. He successfully persuaded local authorities to remove the pump handle, which was followed by a decline in new cases.23
The Grand Experiment: Snow conducted a comparative study of two populations served by different water companies. He found that residents receiving water from the Southwark and Vauxhall Company, which drew from sewage-polluted inlets of the River Thames, suffered significantly higher death rates than those supplied by the Lambeth Company, which sourced water from a cleaner, upstream location.23

These innovative methods, including the use of disease mapping and statistical analysis, provided critical evidence that cholera was transmitted through contaminated water, establishing Snow as the father of modern epidemiology.23