When hundreds of Soho residents suddenly contracted the deadly disease, Snow questioned the predominant theory that cholera was spread by polluted air. (The germ theory, which is the currently accepted scientific theory of disease, had not yet gained traction.)
Snow’s research led him to a communal water pump on Broad Street. He created a map to demonstrate how cholera cases were geographically clustered around the pump. Additionally, his survey of residents determined that people who used other pumps remained unaffected. Snow came to the conclusion that sewage, which was dumped into the River Thames or into cesspools throughout town, could be contaminating the water supply and rapidly spreading disease. He disabled the Broad Street pump by having its handle removed—and the outbreak ended.