smallpox epidemic, proposing that: "If the latent cause of epidemics cannot be discovered, the mode in which it operates may be investigated. The laws of its action may be determined by observation, as well as the circumstances in which epidemics arise, or by which they may be controlled." He showed that during the smallpox epidemic, a plot of the number of deaths per quarter followed a roughly bell-shaped or "normal curve", and that recent epidemics of other diseases had followed a similar pattern. Research on cholera 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. During the 1853-54 epidemic, Farr gathered more statistical evidence. During focused study of the 1854 Broad Street cholera outbreak, the physician John Snow used data supplied by the GRO and applied the (now accepted) mechanism for transmission he had proposed in 1849: people were infected by swallowing something, and it multiplied in the intestines. Snow also examined mortality statistics compiled by the GRO for people supplied with water from two companies in South London – the Southwark & Vauxhall Company (which drew contaminated water from low in the Thames basin) and the Lambeth Water Company (which drew cleaner water from further up the Thames) – and found Southwark & Vauxhall customers were especially likely to suffer. Farr took part in the General Board of Health's 1854 Committee for Scientific Enquiries. The conventional explanation for cholera was still multifactorial; Snow's view of cholera as solely caused by a pathogen was not accepted, though his evidence was taken seriously. Farr's research was detailed and showed an inverse correlation of mortality and elevation. There was a further epidemic in 1866, by which time Snow had died, and Farr had accepted Snow's explanation. He produced a monograph which showed that mortality was extremely high for people who drew their water from