John Snow

John Snow was an English physician best known for identifying the cause of the 1854 cholera epidemic in London. By many he is seen as the founding father of epidemiology, a branch of medicine that studies the contraction, spreading and control of disease. He was also a data pioneer, using carefully plotted maps to solve a problem that had defeated scientists of his day.

Born in 1813, he grew up in York, near the River Ouse, where his father was a coal labourer. Snow soon became aware of the suffering associated with disease. Living in an area with poor sanitation - due to regular floods where drain-waste contaminated the local rivers - provided him with an early insight into how illnesses spread. He became skeptical of the then-prevalent “miasma” theory, the idea that diseases such as cholera were spread through the inhalation of “bad air.”

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After completing a medical apprenticeship and graduating as a doctor of medicine at the University of London, Snow swiftly became a highly successful physician. In 1850 he was enlisted in the Royal College of Physicians, a privilege for only the most exceptional doctors. He was well-known for his proficiency in the use of anaesthetics. Refining the technique by improving the administration process was a big leap forward in surgery, as the relatively new use anaesthesia was still clumsily delivered to patients as a few drops of chloroform on a rag. His patients were the pinnacle of British society: twice, Snow applied his new techniques to Queen Victoria during childbirth. His achievements led to a wider acceptance of the benefit of anaesthesia and a safer, more controlled execution in its use for surgery.

His biggest contribution was his research into the transmission of cholera. In 1848, when Snow first founded the London Epidemiological Society, the common explanation for cholera was miasma theory. Snow however, proposed that cholera was transmitted by water or food that was contaminated by fecal matter. His theory was first tested in 1854 when an outbreak of cholera near Broad Street (now Broadwick Street), Soho, caused 616 deaths. Using maps, graphs, reasoning, and the tracking of hundreds of cases of cholera, he theorised that the disease had first originated from a corrupted water source on Broad Street. By creating a map of all the cholera related deaths in Soho to support his hypothesis, he was able to visualise the area and how the cases of the illness clustered. The map made things clear: there was a deadly problem at the pump.   

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However, due to the fact that there were other water pumps nearby, he had to prove that the cholera was coming only from the pump on Broad Street. Snow created a voronoi diagram to allow him to calculate the spatial relationships between the different pumps and the incidence of the illness. A voronoi diagram is where a plane (such as the area of soho) is split up into a number of shapes. Each of these represents the area around a pump, with the point showing its location.

The area inside the shape is always closer to its own point than in any of the neighbouring shapes. Using this diagram he split the area into different sections, identifying an area around the pump where locals were most likely to retrieve their water.

Today it seems obvious that the outbreaks of cholera were centred around the Broad Street pump. But like Playfair with his charts, Snow did it first; using complex mathematics and a good chunk of common sense to prove his point. This use of data visualisation was one of the founding moments in the history of modern day epidemiology, leading to the removal of the handle to the Broad Street pump.The epidemic ended.

It was years before John Snow’s theories were accepted, partially due to the public’s resistance to the fact that cholera was transmitted through fecal matter. A later investigation into a subterranean well supplying the pump supported the physician’s suspicions. It had been contaminated with cholera, a result of its close proximity to a leaking cesspit full of bacteria. Like Playfair and Petty, he was never recognised for his work in life, but his legacy is huge: the scientific theory of disease has led to substantial changes in sanitation and healthcare across the globe. For us, the fact that he achieved this through what we now call visualisation makes him a true data hero.

by Benjamin Ivens