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 its credit. Many of the diseases which wreaked havoc in the last century have been brought under control, and in the case of smallpox, eliminated, through improvements in hygiene and the use of preventive and control measures such as mass vaccination. Nevertheless, in recent years, new problems have emerged, and old diseases have re-emerged. Many foodborne and waterborne disease outbreaks go unrecognized or are detected too late for effective control measures to be implemented. New infections, such as HIV, present new threats. Antimicrobial drug resistance, particularly the increase in drug resistant TB, also poses new challenges.
