I arrived at the conclusion in the latter part of 1848, that cholera is communicated by the evacuations from the alimentary canal."
--John Snow, On the Mode of Communication of Cholera, 1848
On the Mode of Communication of Cholera (1848) is an essay by English physician John Snow wherein he presents his theory that diseases such as cholera and the plague were not caused by bad air, but by germs that were spread by contaminated water, dirty clothing, and fecal contact. At a time when the germ theory of disease was not yet known, Snow's theory was revolutionary and laid the groundwork for modern epidemiology. In 1855, Snow published a more detailed second edition, which included his investigation of the role of the water supply in the cholera epidemic of 1854 in the Soho neighborhood of London (also available from Cosimo Classics).