John Snow's cholera map. Map of Soho, London, UK, showing deaths (black lines) due to cholera during the epidemic of 1854. This outbreak killed 616 people. 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. This map showed that cases were centred around the water pump on Broad Street (now Broadwick Street). Snow convinced the council to disable the well by removing the handle, leading to a decrease in cases. From On the mode of communication of cholera (1855).