John Snow's Map
John Snow's Map:http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Epidemiology.html

John Snow's map showing cholera deaths in London in 1854. Snow recognized a geographical pattern in the incidence of cholera cases that suggested a hypothesis that he could test. The Broad Street well is marked with an X (in the red circle).

Because of Snow's analysis of the association between the incidence of cholera and environmental factors, he is often considered the "father" of epidemiology: the study of the relationship between the incidence of disease and other factors, e.g., location, exposure to environmental substances, occupation, etc., to find the statistical associations between the disease and possible causes.
Excerpted and adapted from: Epidemiology