showing streets and water pumps to track the incidence of disease. He discovered that
people drinking water originating from the upper regions of the Thames River did
not suffer as high an incidence of cholera as those who were drinking water from the
lower regions contaminated with sewage. He removed the handle of the Broad Street
pump located in the lower region of the Thames River, and the number of cases decreased
gradually.
Snow was instrumental in the application of epidemiological methods such as
identifying common symptoms (case definition), creating maps showing the incidence
and prevalence of disease, recording incidence data with time and place, testing water
sources, communicating with local politicians, scientifically integrating the chain of
events, proposing the mode of transmission of disease (water), and then taking effective
public health action by pulling out the handle of the pump that he suspected to
be involved in disease transmission (Snow, 1991).