In the early 1800’s, John Snow, M.D., a historical figure in epidemiology, provided one of the earliest examples of using epidemiologic methods to identify risk for disease and recommend preventive action. On August 31, 1854, London experienced a recurrent epidemic of cholera and Snow suspected water from the Broad Street pump as the source of disease.

Removal of this handle prevented additional cholera deaths, supporting Snow's theory that cholera was a waterborne, contagious disease. Snow's studies and the removal of the pump handle became a model for modern epidemiology and is considered to be the symbol of when public health started.

This is the original map by John Snow showing the clusters of cholera cases (indicated by stacked rectangles) in the London epidemic of 1854. The contaminated pump is located at the crossroads of Broad Street and Cambridge Street (now Lexington Street), running into Little Windmill Street.

This map is a tremendous contribution to the field of epidemiology, recognizing that part of treating disease requires viewing patients not as individual, isolated cases, but within the larger environment in which they live. Although John Snow’s model is accepted as a way to frame our understanding of infectious diseases today, it gives us a blueprint for approaching illness, particularly illness caused by or related to the patient’s environment.