For several years, a doctor by the name of John Snow had been following thedevastating waves of cholera that hit England from time to time. The diseasearrived suddenly and was almost immediately deadly: people died within a day ortwo of contracting it, hundreds could die in a week, and the total death toll ina single wave could reach tens of thousands. Snow was skeptical of the miasmatheory. He had noticed that while entire households were wiped out by cholera,the people in neighboring houses sometimes remained completely unaffected. Asthey were breathing the same air—and miasmas—as their neighbors, there was nocompelling association between bad smells and the incidence of cholera.

Snow had also noticed that the onset of the disease almost always involvedvomiting and diarrhea. He therefore believed that that infection was carried bysomething people ate or drank, not by the air that they breathed. His primesuspect was water contaminated by sewage.

At the end of August 1854, cholera struck in the overcrowded Soho district ofLondon. As the deaths mounted, Snow recorded them diligently, using a methodthat went on to become standard in the study of how diseases spread: he drew amap. On a street map of the district, he recorded the location of each death.

Here is Snow’s original map. Each black bar represents one death. The blackdiscs mark the locations of water pumps. The map displays a strikingrevelation–the deaths are roughly clustered around the Broad Street pump.

Later it was discovered that a cesspit that was just a few feet away from thewell of the Broad Street pump had been leaking into the well. Thus the pump’swater was contaminated by sewage from the houses of cholera victims.

Snow used his map to convince local authorities to remove the handle of theBroad Street pump. Though the cholera epidemic was already on the wane when hedid so, it is possible that the disabling of the pump prevented many deaths fromfuture waves of the disease.

The removal of the Broad Street pump handle has become the stuff of legend. Atthe Centers for Disease Control (CDC) in Atlanta, when scientists look forsimple answers to questions about epidemics, they sometimes ask each other,“Where is the handle to this pump?”

Snow’s map is one of the earliest and most powerful uses of data visualization.Disease maps of various kinds are now a standard tool for tracking epidemics.

Though the map gave Snow a strong indication that the cleanliness of the watersupply was the key to controlling cholera, he was still a long way from aconvincing scientific argument that contaminated water was causing the spread ofthe disease. To make a more compelling case, he had to use the method ofcomparison.

Scientists use comparison to identify an association between a treatment and anoutcome. They compare the outcomes of a group of individuals who got thetreatment (the treatment group) to the outcomes of a group who did not (thecontrol group). For example, researchers today might compare the averagemurder rate in states that have the death penalty with the average murder ratein states that don’t.

If the results are different, that is evidence for an association. To determinecausation, however, even more care is needed.

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