How One Man's Map Singlehandedly Stopped An Epidemic And Saved Millions Of Lives

Maps have the power to change minds, as the 1861 map that convinced Lincoln to end slavery proves. Maps also have the power to save lives. One such map, the John Snow map, single handedly ended a cholera outbreak and revolutionized epidemiology. Dr. John Snow’s cholera map of London charted deaths in the Soho neighborhood during the 1854 cholera outbreak. Snow’s map proved that the outbreak came from contaminated water at the Broad Street Pump. 

Cholera in London was a horrifying epidemic, made worse by the fact that London routed its sewage directly into the Thames River, the city's source of drinking water. Victims of cholera suffered from horrific diarrhea, and the mixing of fecal matter and drinking water was a deadly combination. 

How did epidemiology start? John Snow, anesthesiologist to Queen Victoria, combined investigative reporting and medical analysis in his cholera map, saving lives and earning himself the title of "father of modern epidemiology." John Snow solved the riddle of cholera as the outbreak struck his own neighborhood.


  • Cholera Was A Terrifying Epidemic That Could Kill Within Hours

    Cholera Was A Terrifying Epidemic That Could Kill Within Hours
    Photo: Wellcome Images / Wikimedia Commons / CC BY 4.0

    When cholera first struck England in 1831, doctors were confounded. They could not agree on the disease's cause, its cure, or even if it was infectious. But everyone knew it was deadly.

    Cholera was dangerous because it quickly dehydrated victims who were struck with uncontrollable vomiting and diarrhea. As The London Gazette reported in 1831, an infected person had a look of “terror and wildness... The skin [was] deadly cold and often damp,” and the tongue was “flabby and chilled like a piece of dead flesh."

  • The Working Theory At The Time, The "Miasma Theory," Blamed Cholera On London’s Slums

    Cholera killed tens of thousands in England between 1831 and 1866, particularly in London’s slums. Many believed that cholera was spread by poisonous air and rotting organic matter, known as the miasma theory of disease. When wealthy Londoners saw the disease’s deadly effects, they blamed it on the pollution and overcrowding in the slums.

    London’s Sanitation Commissioner, Edwin Chadwick, argued that “all smell is disease,” and bad smells spread infections. From this, he concluded that it was more important to air out the slums than purify the drinking water. He also blamed the poor for the issue, to an extent. An 1831 Board of Health report in the London Gazette "attributed cholera to, ‘the poor, ill-fed, and unhealthy part of the population, and especially those who have been addicted to the drinking of spirituous liquors, and indulgence in irregular habits.'" 

  • Doctor John Snow Questioned The Miasma Theory

    John Snow was the son of a coal yard laborer who walked 200 miles to attend medical school, traversing all over the Southwest of England on his way from York to London. At age 18, he witnessed the horrors of a cholera outbreak in Newcastle, where he saw sick men hauled up from the coal pits “after having had profuse discharges from the stomach and bowels, and when fast approaching to a state of collapse."

    After training to become a medical doctor, Snow lived through another cholera outbreak in 1848 that killed between 50,000 and 70,000 people in England and Wales. Snow used new city-wide statistics on death rates to destroy the miasma theory of cholera transmission.

  • Death Statistics Pointed Snow Toward A New Theory: Water

    Death Statistics Pointed Snow Toward A New Theory: Water
    Photo: Wellcome Images / Wikimedia Commons / CC BY 4.0

    If the miasma theory was right, one group would have the highest mortality rate: the night soil men, who literally shoveled human feces for a living – an odious but relatively well-paying Victorian occupation. And yet, John Snow realized their death rate was low. He began to chart the neighborhoods where the highest numbers of people were dying. To his surprise, he saw that the rate of infection was three times higher on the south banks of the Thames versus the northern banks. 

    Snow’s investigation eventually uncovered the difference between these areas: south of the Thames, the water company drew its drinking water downriver from London’s sewage dumping. That convinced Snow that cholera was transmitted through infected water rather than miasma in the air.

  • In 1854, Snow Tested His Theory When Cholera Broke Out In Soho

    In 1854, Snow Tested His Theory When Cholera Broke Out In Soho
    Photo: John Snow / Wikimedia Commons / Public Domain

    In the first five days of the 1854 cholera outbreak, 500 people in London's Soho neighborhood died. Snow, who lived in Soho, leapt into action to test his theory that water spread cholera. He used a list of 83 deaths reported by London’s Register of Deaths to chart the disease. And since he lived only a few blocks east of the outbreak, Snow knocked on doors to ask dozens of people where they got their drinking water. 

    Within 24 hours, Snow determined that the deaths were clustered around the Broad Street Pump

  • Malt Liquor Saved Lives In The Lion Brewery

    Malt Liquor Saved Lives In The Lion Brewery
    Photo: Comenius of Reutlingen / Wikimedia Commons / Public Domain

    The Broad Street Pump stood just outside the Lion Brewery, but not a single worker in the brewery fell ill with cholera. John Snow visited the brewery to confirm his theory that the Broad Street Pump was contaminated, and the owners told him that the workers never drank water – they only drank a daily ration of malt liquor.

    Similarly, the nearby St. James Workhouse had 535 residents but only two died – because they had a private water supply and did not use the public pump. Snow was also able to link seemingly disconnected deaths to the Broad Street Pump by showing, for example, that three schoolchildren had stopped at the pump on their way to school and then died from cholera.