John Snow's demonstration of the spread of cholera through contaminated water, which led to the removal of the pump handle in London's Broad Street during the cholera epidemic of 1854, eventually saved many lives and is rightly celebrated. Yet without the diligence and openness of William Farr, the chief compiler of statistical abstracts in the General Register Office, Snow would not have been able to prove his theory. He relied on Farr's Weekly Returns to make his famous map of cholera deaths around the Broad Street pump. On September 5, 1854, when the epidemic was at its height, it was Farr that Snow went to see to examine the recorded deaths for that week. Three days after their meeting, the pump handle was removed.
Grassly NC. Infectious Disease Surveillance. JAMA. 2008;300(13):1591-1592. doi:10.1001/jama.300.13.1591-b