Snow’s work on cholera demonstrated fundamental intellectual steps that must be part of every epidemiological investigation. He began with a logical analysis of the available facts, which proved that cholera could not be caused by a ‘miasma’ (emanations from rotting organic matter) as proposed in a theory popular at that time; but must be caused by a transmissible agent, most probably in drinking water. He confirmed the proof with two epidemiological investigations in the great cholera epidemic of 1854. He studied a severe localized epidemic in Soho, using analysis of descriptive epidemiological data and spot maps to demonstrate that the cause was polluted water from a pump in Broad Street. His investigation of the more widespread epidemic in South London involved an inquiry into the source of drinking water used in over 700 households. He compared the water source in houses where cholera had occurred with that in others where it had not. His analysis of the information about cases and their sources of drinking-water, showed beyond doubt that the cause was water that was being supplied to houses by the Southwark and Vauxhall water company, which drew its water from the Thames down-river, where many effluent discharges polluted the water. Very few cases occurred in households supplied with water by the Lambeth company which collected water upstream from London, where there was little or no pollution.

This was a remarkable feat, completed 30 years before Robert Koch identified the cholera bacillus. Snow published his work in a monograph, On the Mode of Communication of Cholera (1855). This book has been reprinted in modern editions and is still used as a teaching text.