Who is John Snow?

Regrettably, I must inform you that the subject of today’s post is not Jon Snow, the sensitive warrior who has won the hearts of Game of Thrones fans the world over. If that is why you are here, I apologize and offer you this photo as a token of good faith.

jon-snow-game-of-thronesAww, he even likes puppies! 

Who is John Snow? More accurately, who was John Snow, and why does he warrant so much attention? A doctor who lived in London, England from 1813-1858, John Snow is often credited as one of the fathers of epidemiology, the medical field that studies how diseases spread and how they can be controlled. John Snow’s discoveries laid the foundation for germ theory, which recognizes that the majority of diseases are caused by microorganisms in one form or another.

John_SnowThe REAL John Snow may not have had curly locks, but at least he had the mutton chops to make up for it

In 1854, London authorities faced a crisis. A severe outbreak of cholera in the Soho district had claimed more than 500 lives and was spreading fast. As Snow put it, what they faced was “the most terrible outbreak of cholera which ever occurred in this kingdom”. The cause, however, was unknown, and a solution was nowhere to be found. The theory at that time was that diseases such as cholera and bubonic plague were spread through ‘bad air’. Poor neighborhoods smelled bad due to lack of proper sanitation, and it was this ‘stench’ that was the cause of sickness. It’s important to note that at that time, upper class homes and neighborhoods had piped drinking water and sewer systems, which emptied into the Thames River. Poor neighborhoods used communal pumps as their drinking water supply and did not have developed sewers.

Today we know that cholera is a bacterial infection that accounts for 100,000 deaths per year globally. The bacteria is ingested through contaminated food or water and causes diarrhea and vomiting, leading to rapid dehydration. In severe cases, the illness can turn deadly within several hours. Cholera breakouts have been traced to fecal contamination of drinking water, and thus it can be managed effectively through proper sanitation. Most of the cholera cases today are in third world countries where high population density, latrines and sewers, and unprotected drinking water sources are found in close proximity to one another.

Now Snow had his doubts about the ‘bad air’ theory. In an attempt to find the source of the outbreak, he created a map showing the distribution of cases.

snow_map_detailThis cutout shows just one region from Snow’s full map. The thick black lines represent the number of cases at each location (Source: University of Delaware)

A single look at Snow’s map identifies the Broad Street pump as the obvious culprit. Based on what we know today, this scenario makes logical sense. A contaminated communal drinking water source could expose an entire neighborhood to the pathogen. Since the bacteria must be directly ingested to spread the disease, we would expect the outbreak to be localized to the immediate vicinity of the pump. Snow’s observations align perfectly:

“On proceeding to the spot, I found that nearly all the deaths had taken place within a short distance of the [Broad Street] pump. There were only ten deaths in houses situated decidedly nearer to another street-pump. In five of these cases the families of the deceased persons informed me that they always sent to the pump in Broad Street, as they preferred the water to that of the pumps which were nearer. In three other cases, the deceased were children who went to school near the pump in Broad Street”.

450px-John_Snow_memorial_and_pubToday, a monument has been constructed at the place of the original pump. And, in the typically English style, John Snow pub lurks sneakily in the background.

Snow had hit the mark. Although his theory didn’t quite convince the authorities that the cause of the outbreak was due to bacteria from fecal material making its way into the drinking water supply, it was enough proof for the pump to be shut down and the epidemic to burn itself out. Further studies since have found the remains of a cesspool or primitive septic tank three feet away from the pump, which was paved over and forgotten during widening of the street. Snow’s map and analysis laid the foundation for germ theory, which would go on revolutionize the fields of medicine and public health.

Be warned: you may have just sustained a lethal dose of mostly harmless science If you enjoyed this article, please comment below and share with your friends! I’d love for you to follow me on WordPress or on Twitter @harmlessscience (just click Follow on the right sidebar). Thanks for reading!

Cover photo courtesy of the University of Pittsburgh. Other images from Wikimedia Commons.

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2 responses to “Who is John Snow?

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