John Snow's map of the water supply in London. Map of south London, UK, showing the areas served by the Southwark and Vauxhall Water Company (green) and the Lambeth Water Company (red). In 1852 the latter changed it supply to Thames Ditton, where the water wasn't contaminated by London sewage. The Southwark and Vauxhall Water Company sourced their water from the Thames near London Bridge. During the epidemic of 1854 there was a mortality rate of 71 deaths per 1000 people in the green area, but a rate of only five deaths per 1000 in the red area. At the time it was thought cholera was caused by 'bad air'. However, the British physician John Snow (1813-1858) believed it was spread by contaminated water. His thorough investigations showed that it was water-borne. From On the mode of communication of cholera (1855).