Ignorance is a Killer

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Take the journey of the bacterium that caused all the trouble, Vibrio cholerae. From its source in the Ganges. Jones follows its progress to Broad Street and beyond, to the megacities of today's developing world via some illuminating detours. He details, for example, how the human body's adaptability to alcohol, a pretty good option when clean water is hard to find, became a genetic advantage and helped drive urbanization.

At the time of the outbreak, London's population had tripled in just 50 years, there was nowhere to bury the dead, the sewage system leaked, the cesspools overflowed, and the whole lot drained into the Thames, the source of most of the city's drinking water. "A lot of people looked at the mess," says Johnson, "saw how cholera could wipe out 10% of the neighborhood in six days, and concluded that London would go the same way as ancient Rome."

To be fair, the public-health authorities had begun works to improve the city's infrastructure. But their refusal to retreat from the miasma theory, and the extra piles of atmospheric data they commissioned to prop it up, cost thousands more lives. Snow's victory over the miasmatists wasn't conceded until years after his death in 1858. Some never relented, such as London's otherwise enlightened Health Board chief, Edwin Chadwick, who went to his grave still propounding that belief in 1890 — seven years after the German scientist Robert Koch definitively identified Vibrio cholerae.

Even so, Snow's map, says Johnson, "was a transformative moment in the history of cities." And it was a remarkable success for "consilient" thinking, a philosophy formulated in the 1840s in which a cohesive theory from one discipline is used to inform ideas from another. Snow's work on administering ether as an anesthetic convinced him cholera was ingested, not inhaled. As a physician he understood how disease spread through a body. As a resident of Soho, he had the local knowledge to realize that the contagion radiated from a single source. "When you look at cholera at any one of those levels, it's hard to see how it works," says Johnson. "But here was somebody thinking at the level of microbes, the human body and the city simultaneously, and making connections between them."

One legacy of that process lies hidden beneath London's streets: 132 km long, the world's most elaborate sewer, which began construction in 1858, was completed in little more than a decade and is still a vital defense for a city that has not recorded a single outbreak of cholera since 1866. It's a testament to Victorian England's ability to construct grand solutions to big problems. That's a skill the modern world could use, says Johnson, noting that some 2 billion people are still at risk because they do not have access to clean water. "Unlike hiv or global warming, this is a problem we fundamentally know how to solve," he says. Failure to do so is a scandal worth kicking up a stink about.