Parts of medical history read like detective novels.  The discovery of the source of cholera by Dr. John Snow in London in 1854 is one of those episodes.  The Ghost Map tells the story of Snow's pioneering work in what have now become standard epidemiological methods.  Tracing a cholera outbreak to a local pump in a poor section of London involved many door-to-door visits working with people who weren't always cooperative, incurring the suspicion and/or ridicule of both them and the medical professionals with whom he worked.  In the course of the story the author offers reflections on the organization of cities and on public hygiene.  Snow, an out-of-the-box thinker, also helped develop surgical anesthesia.