In 1831 John Snow was 18 and an apprentice physician in London when a cholera epidemic broke out. At the time, there were no antibiotics and the germ theory of disease had not been accepted. Cholera was believed to be caused by clouds of toxic foul-smelling gases rising from sewers, garbage dumps or gravesites. Patients were treated with ineffective methods such as bleeding, laxatives, peppermint, opium or brandy. The next year the epidemic ended as unexpectedly as it began, leaving 50,000 people dead throughout Great Britain.
Doctor Snow went on to finish his degree and became a physician. He began to suspect that diseases may be caused by tiny invisible parasites that spread from person to person. In 1848 another outbreak of cholera swept through the city of London. He decided to track the spread of the disease.
The first victim had arrived by ship and stayed in a hotel room where he fell ill from cholera and died. The physician who had treated this patient returned to the same room a few days later to treat another cholera patient. This led Dr. Snow to suspect that germs had remained in the room and infected the second patient. As more cases developed, he determined that their digestive symptoms indicated they were ingesting contaminated food or water. He suspected that the severe diarrhea associated with the disease was the means by which the germs were spread. He postulated that waste water poured into channels on the street in front of people’s homes had found its way into the source of drinking water.
He investigated further, going from house to house to determine where people were getting their drinking water. Two companies provided water sourced from the Thames River, one that extracted the water downstream from where a sewage outlet was located and another that used a source upstream from the sewage outlet. His findings showed ratio of 71:5 patient deaths pointing to sewage-contaminated water being associated with death from cholera. Shockingly, the medical community was not swayed by this evidence, and still believed cholera was caused by miasmas of toxic gas.
Dr. Snow continued to investigate as the epidemic raged on and spread to a neighborhood near his home. He obtained information on 83 deaths that had occurred in the neighborhood. He calculated the distance from their homes to the nearest water pump. 73 of the victims lived closer to the Broad Street pump than any other. Of the other 10 victims, he found that 8 of them had obtained water from the Broad Street pump, either because they preferred that water or had drank from the pump on the way to school.
With this new evidence, he was able to convince the local government to remove the pump handle so people could not obtain water from the Broad Street pump any longer. The cholera outbreak quickly ended.
John Snow is widely considered to be the father of modern epidemiology due to his efforts to determine how cholera was spread, and his use of statistics and mapping methods.
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