This tragedy made one of the world’s great medical experiments possible, ‘one of the most important of all time’, according to Angus Deaton.9 John Snow, a physician in London, thought that cholera was borne by water rather than foul air. He mapped the deaths in detail and found a revealing link. All the cholera cases seemed to originate from the water company that had its inlet downstream of the sewage discharge, whereas no deaths were found among those who got their water from the other company, which had recently moved its inlet to purer water upriver. This convinced the local council to
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