The Ghost Map: The Story of London’s Most Terrifying Epidemic—and How It Changed Science, Cities, and the Modern World is a nonfiction book by Steven Johnson. It was published in 2006 and was named a Notable Book of the Year by the New York Times and a Best Book of the Year by Library Journal and Entertainment Weekly.

The immediate subject of The Ghost Map is the cholera outbreak that took place in London in 1854. The outbreak was concentrated in the Golden Square neighborhood of Soho and claimed hundreds of lives over the span of a week. The first cholera victim in the neighborhood was an infant—known in the book as baby Lewis—whose soiled diaper was flung into the cesspool at the foot of the Lewis household. The cesspool in turn contaminated the nearby Broad Street well, infecting many of the citizens who drank from it.

The book unfolds over the course of a week in September 1854, when the outbreak was at its height. Each chapter of the nine-chapter book, excluding the conclusion and the epilogue, is subtitled with a day of the week. The book follows two men who in different ways were instrumental in uncovering the true source of the epidemic, and who ended up becoming friends and working together. The first man is John Snow, a doctor and renowned anesthesiologist, whose theory (one that is accepted today as medical fact) that cholera was contracted through drinking contaminated water was at odds with the then-popular theory of “miasma,” or that people contracted cholera through breathing in foul air. The second man is Henry Whitehead, a vicar in the Golden Square neighborhood, known for his wide-ranging intellect and gregarious manner. While Whitehead was initially skeptical of Snow’s theory, he ended up accepting it and supporting Snow, persuaded by the accounts of survivors and by the on-the-ground evidence that he saw in his neighborhood.

While the book takes place in real time, following Whitehead and Snow in their attempt to outrace the epidemic, it also travels backwards into the past and forwards into the future. It traces the origins of cholera—a disease that has been around since ancient times and that affects many disadvantaged communities still—and explains the complicated behavior of bacteria. It also speculates on the origins of the Victorian belief in miasma as well as on the difference between the Victorian attitude towards cities as generally foul and unwholesome spaces and the general attitude towards cities today. The book ultimately suggests that Snow and Whitehead were important figures not merely due to their discoveries around cholera, but because they helped to usher in the modern idea of cities as spaces of progress and urbanity. The conclusion and the epilogue to the book celebrate the diversity and inherent creativity of city living, while examining the challenges that city dwellers face today—terrorism, global warming—in light of the cholera outbreak that Snow and Whitehead faced in 1854.