At 6am on 28 August 1854, the city of London struggled to sleep at the end of an oppressively hot summer. But at 40 Broad Street, Soho, Sarah Lewis was awake tending to her feverish baby girl. As she threw a used bucket of water into the cesspool at the front of her lodgings, it marked the start of a cholera epidemic that would consume 50,000 lives in England and Wales - and become a battle between man and microbe unlike any other. Steven Johnson takes us day by day through what happened and re-creates a London full of dust heaps, furnaces and slaughterhouses; where a ghost class of bone-pickers, rag gatherers, dredger men and mud-larks scavenged off waste; where families were crammed into tiny rooms and cartloads of bodies wheeled down the streets. And at the heart of the story is Doctor John Snow: vegetarian, teetotaller, anaesthesiologist and Soho resident, whose use of maps to prove that cholera was spread by water - and not borne on the air as most believed - would bring him into conflict with the entire medical establishment, but ultimately defeat his era's greatest killer. Steven Johnson interweaves this extraordinary story with a wealth of ideas about how cities work, ecosystems thrive and cultures connect. He argues that, with half the planet's population set to be urban, today's megacities could soon be wrestling with the same problems as John Snow and that, just as in 1854, science could be our salvation.