In 1854, London was a city with millions of residents and no sewage system. It was the first time a city had grown so big, and while it had a rudimentary idea of a public health system, this system was based on the "miasma" theory of disease: that illness was the result of smelling bad smells. So it was that London was drowning in its own shit, and so it was that thousands of Londoners were in the business of harvesting, cleaning, moving, exploring and scavenging in shit. Johnson quotes journals and accounts of the day describing unimaginable filth, residents dipping buckets into open, running sewers, then letting the "water" separate out of the excrement, skimming it off and drinking it.
Cholera epidemics are the inevitable outcome of such a situation. One such outbreak took Soho -- a poor, overcrowded neighborhood -- by storm, killing one in ten in the space of a week. In that week, two very different men (a cleric and a scientist) who were both local to Soho pounded the streets, working to extinguish the disease's flames. They struggled against the miasma-obsessed public health administration (whose idea of sanitation was to order all the basement cesspools emptied into the Thames, London's main source of drinking water).
The cleric, Reverend Henry Whitehead, had intimate knowledge of Soho's streets and families, and the scientist, Dr John Snow, had a history of challenging establishment superstitions with empirical research. Together they worked on a map that showed the disease's course through London, and ran the cholera back to the well where it originated. The combination of data-visualization and local knowledge revealed the microbial nature of cholera, years before anyone managed to connect the actual bug with the disease.
Johnson's got a gift for telling human stories in science -- and a healthy respect for cities, humanity's most complicated and magnificent inventions. He's characterized this as the story of the world's largest organism -- the city -- locked in struggle with one of the world's smallest -- a bacterium. That's as good a strapline as any -- it's a dramatic story of a key evolutionary moment in our history, a moment when we could have destroyed ourselves or brought ourselves to the future. Link
Update: Jeff sez, "John Snow published a book on his investigation into the cholera epidemic. His account is a pretty interesting read."