The Ghost Map

Theodore Dalrymple reviews The Ghost Map and the heroism of John Snow during the London cholera epidemic. An excerpt:

Perhaps it was his knowledge of the diffusion of gases that first led him to question the principal theory of cholera epidemics at the time—that they resulted from a gaseous miasma that emanated from some putrid source of decay (of which, of course, Victorian London had no shortage). Any infective gas would become extremely dilute at a short distance from the putative source, he reasoned. The distribution of cases in the 1854 outbreak—or any other outbreak in fact—was not compatible with the gaseous diffusion of the disease.

Snow also took the common view that a disease agent whose principal pathological effect was on the gastrointestinal system probably had been ingested, not inhaled. This was not decisive evidence, of course, but Snow was nonetheless right.

By examining the distribution of cases, starting from the new hypothesis that cholera was waterborne rather than miasmatic, Snow deduced that the epidemic’s source was contaminated water from the Broad Street pump, and he persuaded the reluctant authorities to remove the pump handle so that water could no longer flow. A local clergyman, the Reverend Henry Whitehead, at one time skeptical of Snow’s theory, then proved that the initial patient who began the epidemic (the index case, as epidemiologists call it) had poisoned the pump’s water supply.