In 1854, John Snow was a well-regarded London anesthesiologist, tending to Queen Victoria, among others. He was born in 1813 of humble stock, but through education and intellectual perseverance—he obtained his M.D. degree in 1844—was able to rise to a position of scientific prominence. Snow became interested in the emerging field of epidemiology, especially as it applied to cholera, a disease of unknown cause (attributed thirty years later by Dr. Robert Koch to Vibrio cholerae). Two population-based studies—both occurring in 1854—established Snow's reputation, and focused scientific attention away from the fallacious notion of airborne transmission towards the role of contaminated water in the spread of cholera.

Snow's first study occurred after the government had mandated that water companies along the polluted Thames River should move their inlets upstream where the quality of water was better. One company moved its...