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. In addition, through his analysis of several variables and their association with death from cholera, Farr held the belief that elevation was the major contributor to the occurrence of the disease. He also presented how topographical features are able to prevent certain diseases similarly to immunization.