Residents were stumped as to what was causing the illness and it was only when Dr Snow mapped the addresses of the sick that he noticed that most of those who had been struck by the disease had a unifying characteristic - their nearest accessible water source was the pump located at the junction of Broad Street, now Broadwick Street, and Cambridge Street, now Lexington Street, close to the rear wall of what is now a Sam Smiths pub called the John Snow.
Snow subsequently managed to persuade those responsible to take the handle off the pump, and this prevented infected water from being used. Later on, it was discovered that the spring below had been contaminated with sewage. A replica pump has since been installed nearby (see above) to commemorate this. He also managed to use statistics to show the connection between the quality of the source of water and cholera cases, demonstrating that the Southwark and Vauxhall Waterworks Company was using water from a sewage-polluted section of the Thames and delivering it to homes with an increased incidence of cholera.
For more on Dr John Snow, see http://en.wikipedia.org/wiki/John_Snow_(physician)