John Snow

John Snow mapped out cases of cholera during an infamous 1854 outbreak in London.

In 1854, there was an outbreak of cholera in the London neighborhood of Soho, one of many mid-century epidemics to strike the English capital, then the largest city in the world.

But this episode was different. An English physician named John Snow — born two hundred years ago today — decided to plot the cases on a map of the neighborhood. In doing so, he identified a contaminated well as the source of the illness, founding the study of public health, and changing the design of cities forever.

Today, there are a host of events in Snow's honor. In York, a two-day program commemorates his contributions to epidemiology and anesthesia. (Every year, the John Snow Society honors the doctor in an annual Pumphandle Lecture, accompanied by a ceremonial removing and replacing of a pump handle.)

Robin Wilson at Southampton University has digitized and georeferenced Snow's original data, so that mapmakers can overlay his points on modern-day maps, or compare them with other data-sets. Wilson's data is also the subject of a tutorial over at the mapmaking site CartoDB, where fifteen minutes can turn you into a modern-day John Snow.

Here's one such effort by Simon Rogers and Andrew Hill at The Guardian, which turns Snow's stacking method into a series of discs:

Simon Rogers/Andrew Hill, The Guardian. Map by CartoDB; data from Robin Wilson/John Snow.

In those days, data mapping was a rare thing. Pasteur would not prove the existence of bacteria until the 1860s, and the predominant theory in London was that diseases like cholera were transmitted through the foul air, or miasma. The air was indeed foul, and would later prove to cause its own health problems, but Snow's map indicated that the source of the outbreak was much simpler: all the cases were concentrated around the Broad Street pump, and many of the victims had been known to drink from it.

A close-up from Snow's map of cholera victims, via Wikipedia.

Snow was not the first person to map an outbreak of cholera in London, but he was the most influential.

Snow wrote to the editor of the Medical Times and Gazette,

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...

With regard to the deaths occurring in the locality belonging to the pump, there were 61 instances in which I was informed that the deceased persons used to drink the pump water from Broad Street, either constantly or occasionally...

The result of the inquiry, then, is, that there has been no particular outbreak or prevalence of cholera in this part of London except among the persons who were in the habit of drinking the water of the above-mentioned pump well.

The Broad Street Pump turned out to be in close proximity to a cesspit.

It's hard today to fathom that this was a revolutionary discovery just 150 years ago, but Snow's theories were largely rejected by the medical establishment and the city government. It was not fear of cholera but rather the smell of sewage -- the famous Great Stink of 1858, caused by the city's tens of thousands of water-contaminating cesspits -- that precipitated the construction of London's gargantuan sewer network over the next ten years.

The result, as Snow had foreseen before his death in 1858, was a dramatic decline in epidemics. Over the next fifty years, nearly every large city in the world would construct systems to dispose of sewage and import clean drinking water.

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