A key event in the development of modem epidemiology was the discovery by the English physician, John Snow, that cholera is transmitted by contaminated water. During the cholera epidemic in London in 1854, Snow mapped the locations of cholera deaths, observed a cluster of victims in a particular neighbourhood and found that most of these cases had drunk water from a communal water pump. The handle of the pump was removed at Snow's insistence, and the epidemic ended within a few days. Since these early days, the science of epidemiology has grown into a major discipline, with many successes to...