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| WIKIMEDIA COMMONS |
The modern understanding of infectious disease began what we would now call a mashup.
The Industrial Revolution transformed Britain into a country which was richer, but filthy, dangerous and miserable for the millions who poured into its growing cities. London became the largest city in the world, and the largest city any human society up to that point had produced.
Sanitary arrangements, however, had not caught up the metropolis's vast complexity. Water came from public wells drawing from the city's mysterious innards, while what was euphemistically called 'nightsoil' was allowed to accumulate under houses.
Nightsoil men had to be paid to remove the nightsoil, and property owners who were broke or just indifferent often didn't bother to hire them. Author Steven Johnson, in The Ghost Map, quotes an engineer writing in the 1840s:
"I found whole areas of the cellars of both houses were full of nightsoil to the depth of three feet, which had been permitted for years to accumulate from the overflow of the cesspools ... Upon passing though the passage of the first house I found the yard covered with nightsoil from the overflowing of the privy to a depth of six inches and bricks were placed to enable the inmates to get across dryshod."
From a modern point of view, the results seem inevitable. In the summer of 1854, a devastating outbreak of cholera struck the Soho area of the city.
The outbreak attracted the interest of maverick physician Dr. John Snow, who had an eccentric theory, not widely accepted, that disease could be transmitted through water. The medical establishment of the day had reached a consensus around the miasma theory of disease, in which illness was spread through bad smells.
Snow and his collaborator, local clergyman Henry Whitehead, painstakingly investigated every case of cholera, concentrating on what the victims’ source of drinking water was. All were mapped, with the hardest-hit houses accumulating a sad row of black marks.
The map combined four data sets: the underlying geography, the victims, the pumps, and lines showing the closest pump from any given point – in the area’s winding streets, this was not always the closest as the crow flew. Once the information had been mapped visually, one thing was clear: at the centre of the outbreak was the pump on Broad Street, seen above.
Ironically, the Broad Street pump had been popular because its water tasted and looked cleaner than the water from many other sources.
After the information in Snow’s map forced local authorities to look closely at the well, a decaying brick sewer was found next to it, fouled by what Whitehead described as ‘abominations, unmolested by water, which I forbear to recite.’
Johnson explains:
The “ … investigation had unearthed the gruesome truth. The contents of a cesspool were seeping into the Broad Street well. Anything living in the intestines of the residents of 40 Broad had direct access to the intestines of about a thousand other human beings. That was all the V. cholerae had needed.
Snow’s map had a longer-term influence as well:
As the waterborne theory of cholera became increasingly accepted, the map was regularly invoked as a shorthand explanation of the science behind the theory. It was easier to point to those black bars emanating ominously from the pump than it was to explain the whole idea of microorganisms invisible to the human eye.
The understanding which Snow's map had helped to emerge then led to the enormous project of giving London a recognizably modern water and sewage system.
Snow's mashup is the direct ancestor of the applications for interactive mapping we use today - it's just that our tools are faster and more sophisticated.
This week’s map (link) shows rates of tuberculosis.
Our series on communicable disease begins this week with five sexually transmitted diseases.
This week, we begin a four-week series on infectious disease. I’ve been wanting to come back to mapping public health data since 
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