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Diagnosing disease is like solving a mystery. And when it comes to highly infectious diseases, the need to solve the mystery quickly is that much more urgent in order to prevent death and to arrest the disease’s spread.

The Ghost Map tells of a time when the cause of most infectious diseases was still not yet known. In August of 1854 in London, a virulent strain of cholera killed 700 people within two weeks in a small, densely crowded area in Soho. At the time, most scientists believed that cholera struck particular types of victims — those living in squalor, the poor and those living near the “deadly miasmas” of open sewers and ancient graveyards.

In The Ghost Map, we see a small neighborhood around Broad Street ravaged by the disease, with entire households struck dead within days. Enter Josh Snow, a respected physician who lives a few blocks from Broad Street. Snow is a thinker, tenacious and willing to question medical standards. He and Henry Whitehead, an amateur sleuth and neighborhood minister, interview survivors of the Broad Street outbreak to pinpoint the path of the disease. In doing so, Snow concludes that the disease struck those who drank contaminated water regardless of class or proximity to open sewers or graveyards.

The Ghost Map reveals how extraordinary Snow was for his time. In convincing the local Board of Health of the cause of the outbreak, Snow was one of the first to use a methodical survey of the actual social patterns of the epidemic and the disease’s effect on the human body. Snow also created a disease map which lays out his theory with stunning visual clarity. The map charts the number of deaths by location, drawing the eye to the contaminated water source and overturning decades of mistaken beliefs about cholera.

Snow’s pioneering work has resonated through history. With the cause of cholera properly identified, treatment has become swift and effective. In nineteenth century London, the importance of clean water prompted the city to design and engineer a new sewer system to ensure wastewater was diverted far away from the city’s population. And as for Snow’s map, his masterwork in data visualization is still taught today.