When Prince Albert announced his plans for the Great Exhibition of 1851 in London’s Hyde Park, his speech idealized the times: “We are living at a period of most wonderful transition, which tends rapidly to accomplish that great era to which, indeed, all history points: the realisation of the unity of mankind.”

Mankind — as Steven Johnson points out in his incisive scientific history The Ghost Map: The Story of London's Most Terrifying Epidemic — may have become more unified with urbanization and industrialization, but with the onset and entrenchment in 1854 of the worst cholera outbreak in Victorian London, where over 2 million people were packed into a ten-mile circumference, the results were often far from wonderful.

The transmission of cholera, caused by the bacterium Vibrio cholerae, is facilitated by such population densities and passed along to humans through contaminated water — and not air or smell, the prevailing “miasma theory” having been a hard lesson to unlearn — while the disease commonly progresses from the first liquid stool to shock in four to twelve hours, with death following in 18 hours to several days without rehydration treatment. Indeed, the latter remedy should have constituted the advice being heard by patients instead of the usual “elaborate mix of homespun remedies, commercial elixirs, and pseudoscientific prescriptions.”

Londoners had known the menace of cholera on British soil before 1854, of course. There had been outbreaks since 1831, including the epidemic of 1848-1849 that claimed 50,000 lives in England and Wales. This time around, however — in this “story with four protagonists: a deadly bacterium, a vast city and two gifted but very different men” — things were going to be different after the waste from a single infected infant got into the water supply near the popular Broad Street pump in Soho, allowing V. cholerae to kill hundreds.

Johnson, author of Everything Bad is Good For You and writer for Wired and Discover, skillfully treats the accounts of these "very different men" — Dr. John Snow and Reverend Henry Whitehead — first as they respond independently to the cholera outbreak to seek the source, and then as their paths are increasingly interwoven and single-stranded to a single purpose in walking the streets and mapping the disease to determine who was dying, who was surviving — and where — to solve the mystery of how the cholera spread.