WHILE VISITING LONDON recently, I took my eleven-year-old daughter to a narrow lane once known as Broad Street and now called Broadwick. Doctors and public health mavens of all stripes make pilgrimages to this corner of Soho in honor of John Snow, the man credited with inventing the modern field of epidemiology. It was Snow who toppled the long-standing medical opinion that foul-smelling vapors, or miasmas, emanating from ubiquitous mounds of rotting garbage and feces caused cholera, the most dreaded disease of the nineteenth century. And since this remarkable physician applied a decidedly cartographical approach to solving a centuries-old quandary, his work serves as the centerpiece of Tom Koch’s elegantly written and richly illustrated book.

Snow’s fascination with cholera began when he encountered it professionally in the seaport town of Sunderland, near Newcastle, in 1831. Twenty-three years later, he tracked down the sources of London’s great cholera epidemic of 1854 by meticulously surveying every case, even to the point of checking each home’s water bills. The good doctor discovered that Londoners who drew their water from the Southwark and Vauxhall Water Company, which came from the fecal-contaminated Thames River, contracted cholera nine times more often than those whose water was supplied by the Lambeth Company, which originated from an upstream (and cleaner) source.

Broad Street looms so large in the imaginations of infectious disease experts because of Snow’s subsequent study of cholera’s effect on that avenue. While drawing up detailed maps of the neighborhood, he noticed that some five hundred fatal cases of cholera—which occurred over ten days—arose not more than a hundred yards from a widely used water well. Upon determining that a sewer pipe passed a few feet from the well, Snow convinced the parish councilors to remove the well’s pump handle, thus making it inoperable and the tainted water drawn from it unavailable. And lo and behold, the cholera cases in the neighborhood fell dramatically.

Although Snow hypothesized that a waterborne “poison” capable of self-reproduction was in the excreta and vomitus of the cholera patients and, ultimately, the water supply, Victorian medicine was nowhere close to our modern understanding of cholera as the result of consuming water or food teeming with a comma-shaped bacillus called Vibrio cholerae. Back in 1854, the mere suggestion that tiny microbes were capable of wreaking such havoc would have been laughable to the overwhelming majority of physicians practicing on both sides of the Atlantic Ocean.

No, that discovery would not occur until a few decades later thanks to another man named Koch—Robert Koch, the eminent bacteriologist and co-founder (along with Louis Pasteur and Joseph Lister) of the germ theory of disease. In 1876, Robert Koch conducted experiments demonstrating the bacterial cause of anthrax, a major disease among shepherds and woolgatherers given that sheep enjoy rolling about the pasture and are frequently infected with the spores causing the clinical illness. In 1882, he astonished the world again by announcing he had discovered the cause of tuberculosis, a feisty microorganism called Mycobacterium tuberculosis. In the following year, a severe cholera epidemic struck Calcutta and Bombay before moving westward to Egypt. Robert Koch and his intrepid assistants travelled to Alexandria where they conducted numerous autopsies, cultured every microbe they found, and demonstrated that the gastrointestinal tracts of those who succumbed to cholera were riddled with Vibrio cholerae. It was a Q.E.D. moment of gargantuan proportions.