The Story of John Snow

“Snow’s work was constantly building bridges between different disciplines, some of which barely existed as functional sciences in his day, using data on one scale of investigation to make predictions about behavior on other scales”.

— Steven Johnson

It was in the 1850’s in London, England which at that time was a thriving center for trade and the arts. Many areas of the city were occupied by people who were very poor and were living in crowded conditions.

Parts of the city were very dirty, and diseases would occasionally surface and spread throughout the population and many people would die. One of these diseases was named cholera. The first symptom of cholera was queasiness, followed by stomachache, vomiting, and diarrhea so prolific that it caused victims to die of dehydration. Hundreds of people would die in a week and several thousands of people would die in a month. Cholera was reported to have killed hundreds of thousands of people on the European continent.

People in those days didn’t understand how infectious diseases were transmitted from one person to another. Cholera would show up and then go away. The people were extremely fearful of this disease and were afraid it would come back again and kill them. Below is an illustration from a satirical magazine called Punch. The message was cholera ruled and the people of London were its target.

People in those days believed that the primary cause for diseases being spread were bad smells in the air. Bad smells would arise from sewers, swamps, garbage pits, open graves, and other foul-smelling sites of organic decay The people who believed this theory were called miasmatists because bad smells were called miasmas.

Some people suggested remedies for cholera. One suggestion was to move to where the air was clean. Another was to carry a bunch of flowers in your pocket to override the bad smells. Someone even suggested firing off barrels of gunpowder in the middle of London, so that the smell of the gunpowder would get rid of the bad smells.

Some very well-known people were miasmatists. One was the famous nurse Florence Nightingale, and another was the Commissioner of the General Board of Health. However, one man named John Snow (a physician and anesthesiologist) doubted the miasma theory for cholera. He had noticed that in two neighboring houses, the epidemic would behave quite differently. In one house, everyone would die of cholera and in the house next door no one would get sick. Since the residents of both houses were breathing the same air, John Snow believed that miasmas theory was probably not the culprit. When he examined the symptoms of the disease, including vomiting and diarrhea, he decided that it must be something that the people were eating or drinking. He finally believed that drinking dirty water was probably the cause of cholera.

To confirm his belief, John visualized the problem by drawing a map as shown below.

Each small black rectangle represents one person who died of cholera at a specific address or location. Where there are several black rectangles stacked up, several people died of cholera at a given address. Because John Snow felt that dirty water was the cause of cholera, he also marked the water pumps. You can see in the map that numerous people died of cholera near the Broad Street pump. John came to believe that drinking water from the Broad Street pump was causing the spread of the disease.

There was another brewery, close to the Broad Street pump on Broad Street, that showed no black rectangles and no deaths. Upon further investigation it was found the people in the brewery drank what they brewed. If they needed water the brewery had its own well. A similar finding was at the workhouse, where there were a lot of poor people and very few deaths. Why? The workhouse had its own well where water was pulled from. It was also found that some of the deaths in regions far away from the pump were those of children who drank from the Broad Street pump on their way to school.

There were two cases that were initially problematic to John. There were two ladies, an aunt and her niece, who died of cholera that lived in Hampstead that was far away from London. Upon further investigation, John Snow discovered that they had actually lived in London and moved to a more refined area called Hampstead to be protected from cholera. However, they did not like the taste of the water in Hampstead, so they had water brought to them from the Broad Street pump on a daily basis.

After making these observations, John was confident of the association between drinking dirty water and getting cholera and he wanted to take action. He proceeded to ask the local municipal authority to disable the handle of the Broad Street pump. After the handle of the pump was removed, people could no longer drink from the pump. The result was that the number of deaths from cholera decreased significantly. It was believed that John may have saved several thousand lives from this disease.

Shown below is the current area on Broad Street in London with the water pump without the handle and the John Snow pub, named after the famous doctor.

John Snow was a scientist and he knew that while he had a strong case for association, he had not yet established causality. To establish causality, he had to do more investigations and gather more evidence.

To confirm that drinking dirty water caused cholera, John used another map of a region of London (shown below).

Water to this region was supplied by two water companies, Southwark & Vauxhall (S&V) and Lambeth. The Thames River runs through London. S&V pulled its water downriver, from where sewage was discharged into the river, so its water was contaminated with whatever was coming out of the sewage pipes. The Lambeth company’s water was drawn upriver from where the sewage was discharged, so its water did not contain sewage waste.

John Snow looked at this area of London where both companies supplied water to different houses, and he compared the number of death rates due to cholera in the houses that were supplied with water from each of the companies. In what he called his grand experiment, he wrote, “There is no difference whatever in the houses or the people receiving the supply of the two Water Companies, or in any of the physical conditions with which they are surrounded.” The only difference was the water company that supplied water to the households. John concluded that if he saw differences in the outcomes for the houses supplied by the two companies, then those differences could be attributed to the water because there were no other differences between the households.

Listed below is John Snow’s table showing the number of houses and cholera deaths for each water supply company. Recall that S&V was the company that pulled its water downriver from the sewage discharge and Lambeth was the company with the cleaner water.

S&V had 1,263 cholera deaths and Lambeth had 98 cholera deaths. However, S&V served many more houses than Lambeth. When you adjust for total number of houses for each water supply company, S&V has 315 deaths per 10,000 houses and Lambeth has 37 deaths per 10,000 houses. S&V has almost 9 times the number of cholera deaths as Lambeth.

It is interesting to note that in John Snow’s lifetime, the English medical establishment did not accept his reasoning. The miasmatists theory seemed to overrule the logical explanation.

While John Snow was looking at the evidence from the 1854 cholera outbreak, another scientist named Filippo Pacini from Italy had identified Vibrio, the bacterium that causes cholera, but nobody listened to him either, because Italy had its own miasmatists. Decades later, Vibrio was identified in Germany, and people had a better understanding of how diseases spread.

Snow’s experiment was key to establishing causality by comparing the number of cholera deaths in houses where people consumed water from two different water supply companies and looking at the differences between the two sets of results. Snow’s brilliant studies of cholera in 1854 earned him the title “the father of modern epidemiology.” His work led directly to steps taken to improve water safety in London, setting new standards for other urban centers across the industrialized world, resulting in cholera, typhoid, and other infectious diseases largely disappearing in many countries and saving millions of lives over the years.

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