One stifling, humid day in August 1854, residents of a London neighborhood began falling violently ill. Within three days, 127 people had died in a sudden outbreak of cholera. A week later, the toll had risen to 500.
For years, a local physician, John Snow, had been convinced that cholera was caused not by “miasma in the atmosphere,” as was then believed, but by contaminated water. As soon as he learned of the outbreak, he began patrolling the neighborhood, seeking patterns. He reviewed death records, interviewed victims’ families, and plotted the cholera cases on a map.
Clues soon emerged. Nearly all the deaths, Snow realized, clustered around the water pump on Broad Street. The closer people lived to another pump, the less likely they were to have contracted cholera. Only a few of the more than five hundred inmates of a nearby workhouse fell ill; the workhouse had its own pump. None of the workers from a neighborhood brewery, where the men drank free beer rather than water, became sick. One final clue—the death of a widow who had not visited the neighborhood for years—confounded Snow until her son revealed her daily habit of dispatching a servant to the Broad Street pump, for she preferred the taste of its water. Although the epidemic had already begun to subside, Snow convinced the local parish to remove the handle from the pump, and the outbreak ended.
More than a century and a half later, scientists at Silent Spring Institute are harnessing the power of mapping to untangle clues to another health crisis—the breast cancer epidemic. They realize that their work must be painstaking for the clues to accumulate and the patterns to form, especially since a tumor may develop years or even decades after a woman’s initial exposure to a carcinogen. So they are employing a range of tools that allow them to overlay different sets of data across geographies and through time.