Christopher Hamlin reviews The Ghost Map: The Study of London’s Most Terrifying Epidemic—and How It Changed Science, Cities, and the Modern World by Steven Johnson, and The Strange Case of the Broad Street Pump: John Snow and the Mystery of Cholera by Sandra Hempel, in American Scientist:
To epidemiologists, the London doctor John Snow (1813-1858) is no mere pioneer—he is an icon for the discipline, whose still-cited work represents a common core of method and rigor. In the treatise for which he is famous, On the Mode of Communication of Cholera (1855), Snow elucidated the means by which the disease was spread during the London epidemics of 1848-1849 and of 1853-1854: through fecal-oral transmission of a specific pathogenic agent in contaminated water. He reached this conclusion chiefly on the basis of two natural experiments.
First was the investigation Snow made in the summer of 1854 of an area of south London served by two water companies, one using an upstream source, the other drawing from the sewage-ridden tidal Thames. Because these rival companies had at one point competed head to head, some streets had beneath them mains from both companies, with adjacent homes relying on one or the other for service. Such conditions permitted something like an accidental randomization of every variable except water source. But Snow found profound differences between the two companies (nearly an order of magnitude, he claimed) in the number of cholera deaths per household served.
Better known is Snow’s mapping of cases of cholera in Soho near the Broad Street pump, a hand-operated affair that served up drinking water from a shallow well. There Snow focused on a sudden eruption of cholera within a single densely populated neighborhood. He showed that use of water from the Broad Street pump was a common factor in almost all of the cholera deaths and also that nonuse of that water was a characteristic of two groups (workhouse residents and brewery workers) that suffered little from the disease. In likening the behavior of the apparent cholera agent to a living thing, Snow is often listed as a pioneer of the germ theory. Empirically, he predicted the characteristics of Vibrio cholerae, the organism that Robert Koch would identify almost three decades later (and which Filippo Pacini had described much earlier, at about the same time that Snow was carrying out his investigations).
More here.