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
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
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.