This is the second part of my blog series.
In the medical community, Dr. John Snow is considered the father of modern epidemiology. He is known for successfully investigating the cause of a cholera outbreak in London in 1854. Through careful surveying of the deceased individual’s location of death, and which company was their water provider, Dr. Snow noticed that the locations of the deceased seemed to correlate to where they obtained their water. Contrary to popular retelling, Dr. Snow did not figure out the cause of cholera from looking at a map of data. Before the Broad Street pump incident, Dr. Snow was already investigating two water companies that served London; the Southwark & Vauxhall Company and the Lambeth Water Company. An outbreak of Cholera in the Soho area of London occurred in 1854 which offered him the opportunity to test his hypothesis: that cholera was transmitted via water and not air as believed by miasmatists. Miasmatists believed diseases were caused by bad smells. Dr. Snow plotted the locations of the disease in the Soho part of London and observed a pattern emerging. The pattern seemed to surround one water source: the Broad Street water pump outside 40 Broad Street.
Illustration 2: Cholera Death Distribution
The illustration 2 map shows the locations of people who died from cholera in red and their location of death. Looking closely at the map you notice the cluster of diseased individuals at the center of the map with the Broad Street pump in the epicenter. There appears to be at least one individual dead per building within a ten-building area, except for one building near the pump that is an outlier. A good surveyor will spot outliers and begin the detective work to find out why the outlier is there in the first place.
Dr. Snow performed this analysis without the benefit of germ theory, which would not be invented for another 30 years after his death. He never actually saw the disease pathogen, cholera. He did not have to fully understand the pathogen, he merely needed to understand its transmission and what was allowing it to be transmitted. He was able to make a recommendation to remove the Broad Street pump handle to stop the spread of cholera, despite not having 100% evidence. Dr. Snow had an extremely hard time convincing the London General Board of Health that the Broad Street pump was the source of cholera. He was fighting something more difficult than Cholera: the medical communities hard-held beliefs. The commonly held medical belief at the time was that disease came from bad air. The London General Board of Health believed in this bad air theory called miasma. Miasma was considered a kind of gas that when consumed would render someone sick, but only if their bodies were susceptible.
In order to improve his evidence to fight the miasma advocates, Dr. Snow needed to find outliers that disproved the miasma theory and supported his water-borne disease theory. He began his methodical search for outliers; one of the outliers he found was Susannah Eley. Susannah Eley was another victim of cholera, she happened to reside far away from the Broad Street pump and "the presumably diseased smells." She shared the common trait of drinking the Broad Street water, which her son diligently delivered to her, because she had grown accustomed to it. Susannah Eley’s niece had visited her aunt and also drank the Broad Street pump water after which she died from cholera. Dr. Snow would go on to ask, “How did Susannah and her niece become the only recipient of the miasma so far away and yet others in her neighborhood did not get cholera,” and “what was so different about their physical constitution?”
Dr. Snow discovered another outlier: a brewery near the Broad Street pump. None of the employees who worked at the Lion brewery became infected with cholera like the rest of the community. If the miasma theory was correct that bad air caused cholera, how did this group of people not contract cholera when they were in the center of the outbreak area? Upon surveying the employees at the brewery, Dr. Snow discovered that the employees of the brewery either drank the beer from the brewery, or drank the water from a separate well the brewery had access to. This surveying approach of looking for the commonalities, differences, and outliers helped Dr. Snow generate the data necessary to convince the General Board of Health to accept his recommendation to remove the Broad Street pump handle, although they did so reluctantly. He later went on to show that the water companies in London exhibited differing levels of customer contraction of cholera, based on the company's water intake location. It turned out that clean water, free of sewage, was better than dirty water. Dr. Snow eventually proved, without a doubt, that cholera was transferred via water, not air. This may not seem like a great feat today with all our technology and scientific processes, but it bears mentioning that Dr. Snow never saw the bacterium Vibrio Cholerae. He never saw his enemy; he did not have the opportunity to see it, to dissect it, or reverse engineer it. He simply analyzed the commonalities, differences, and outliers in the environment and hosts. With this limited information he was able to recommend a simple control mechanism to stop the spread of this deadly pathogen. If we take a page from Dr. Snow's playbook, we do not always have to know everything about the digital disease pathogen to institute an effective mitigating control. If we learn enough about the digital diseases in our environment we can institute our own pump handle removal recommendation.
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