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In August 1854 , a cholera outbreak tore through the Soho district of London . More than five hundred people died within ten days . The speed of the deaths terrified the neighbourhood . A local doctor named John Snow decided to investigate . He believed that cholera spread through dirty water , not bad air . He collected the addresses of the people who had died and talked to their families . Most of them had drunk water from a single public pump on Broad Street . On 7 September , Snow took his evidence to the local Board of Guardians . The next day , the board had the pump 's handle removed . The outbreak was already slowing by then , as Snow himself admitted . Even so , the closed pump could not cause a second wave of illness . The famous map came a few months later . Snow drew each death as a small black bar at its address . The bars crowded around the Broad Street pump and grew rare further away . The map did not change official thinking at once . Most authorities still believed that cholera came from bad air . The water theory won only years later . Even so , the Broad Street map became a founding document of epidemiology . It showed that the place where illness appears can reveal its source . Public health workers still map cases every time a new disease emerges .
