There was a major outbreak of cholera in London in 1849 which killed around 15,000 people. Early industrialisation had made London the most populous city in the World at the time, and the River Thames was heavily polluted with untreated sewage. Farr subscribed to the conventional theory that cholera was carried by polluted air rather than water – the miasmic theory. On the other hand, as a result of studying this outbreak, the physician John Snow proposed what is now the accepted mechanism for transmission: people were infected by swallowing something and it multiplied in the intestines.