You see, the housing at this point was very compact due to the amount of people that came flooding into London because of the amount of jobs that had been created with the industrial revolution. This caused lots of compact 'blocks' of houses to be built, if you would imagine a square of houses with a courtyard in the centre. Many of these houses did not have sanitation and therefore used their cellar as a toilet, which were cleared out by gong farmers (as I have said in my hub). Some of these blocks had one water closet in the courtyard for all the families to use and often the pipes that carried the sewage to the thames (as sewers were not built untill 1865) were terribly close to the single water pump, shared by the whole block, the sewage would often leak into the water pipe, introducing bacteria into the water supply. John snow saw that Cholera only affected certain areas, and in those areas, one block of houses may have had many cholera cases while one next door to it would have had none. He decided to investigate the water supply as a possible cause and stopped the use of a water pipe which supplied an affected block, they had to get water from a clean supply and teh cholera cases plumeted, thus proving cholera to be a waterborn disease. This was indeed one of the main 'wake-up calls' that caused the government to majorly improve housing and through that, public health in London.