The English anesthesiologist John Snow, whose 200th birthday is being celebrated this month, is considered one of the pioneers of the field of epidemiology for his innovative use of a data mapping to find the sources of a cholera outbreak that swept through London's Soho district in the 1850s.
At the time, the link between cholera and drinking water was not understood and the spread of the disease was attributed to unclean air. By mapping the fatalities from the disease, Snow noticed that they were clustered around a particular water pump on Broad Street. And the outliers only bolstered his case.
The Guardian's Simon Rogers explains:
One 59-year-old woman sent daily for water from the Broad street pump because she liked its taste. Wrote Snow:
I was informed by this lady's son that she had not been in the neighbourhood of Broad Street for many months. A cart went from broad Street to West End every day and it was the custom to take out a large bottle of the water from the pump in Broad Street, as she preferred it. The water was taken on Thursday 31st August., and she drank of it in the evening, and also on Friday. She was seized with cholera on the evening of the latter day, and died on Saturday
At a local brewery, the workers were allowed all the beer they could drink - it was believed they didn't drink water at all. But it had its own water supply too and there were consequently fewer cases.
In nearby Poland street, a workhouse was surrounded by cases but appeared unaffected: this was because, again, it had its own water supply.
It turned out that the water for the pump was polluted by sewage from a nearby cesspit where a baby's nappy contaminated with cholera had been dumped. But he didn't just produce a map; it was one part of a detailed statistical analysis.
Snow's map was an ancestor of the epidemiological maps used to track disease outbreaks today -- including recent cholera outbreaks -- but as Rogers notes, Snow's map probably wouldn't pass muster with analysts today. For one thing, it doesn't take into account the population of the are -- Broad Street may have had the most deaths because it had the most people. But at the time, it was good enough to convince the authorities to disable the well -- helping to end the outbreak.
An exhibition in honor of Snow and his contributions to disease mapping is currently being held in London.