In late 1854, a doctor was called to a house in Hampstead, London. He found the patient - an elderly lady - scrunched up into a ball, a rictus of agony contorting her features. More remarkably, all the moisture in her body seemed somehow to have been sucked out, leaving her dried up like a shrivelled husk. Of all the diseases that afflicted people in the nineteenth century, the pandemics of cholera were undoubtedly the most terrifying. Yet much of their horror could have been avoided for, at a comparatively early stage, their true nature and course had been uncovered. In one of the most extraordinary detective stories in the history of scientific discovery, an English doctor had patiently tracked down the disease, and unmasked it for what it was. This book, written for the general reader, tells how John Snow used a mixture of observation, logic and statistical evidence to go against the overwhelming weight of medical opinion and arrive at a correct understanding of the sickness.