Megalopolis and micro-organism

The story of Dr John Snow’s investigations into the causes of the cholera epidemics in mid-Victorian London has been written up several times, most recently in a book by Sandra Hempel which I reviewed in these pages six months ago. So do we need yet another account of them? Perhaps not, except that Steven Johnson approaches them from an unusually interesting angle: his book is less concerned with medical history than with urban history and, in particular, the rise of mega- cities. ‘We are now, as a species,’ he claims, ‘dependent on dense urban living as a survival strategy.’

If you’re wondering what relevance Snow’s discovery, dismissed and mocked as it was at the time, that the culprit in the devastating 1854 outbreak of cholera in Soho was the water pump in Broad (now Broadwick) Street has to this large claim, Johnson’s answer is, ‘Broad Street marked the first time in history when a reasonable person might have surveyed the state of urban life and come to the conclusion that cities would some day become great conquerors of disease. Until then, it looked like a losing battle all the way.’ In other words, Snow’s pioneering work in epidemiology, epitomised by his map of Broad Street and its environs that gives Johnson’s book its title, was an essential component in paving the way to cleaner, healthier cities.

The Ghost Map’s opening sentence sets the scene: ‘It is August 1854, and London is a city of scavengers.’ Johnson evokes the world of Henry Mayhew and lists no less than 11 trades that are all forms of scavenging. He emphasises the crucial role played by the poorly rewarded ‘pure- finders’ and ‘night-soil men’ in recycling the city’s waste, but points out that London was ‘a Victorian metropolis trying to make do with an Elizabethan public infrastructure’.