Epidemiologists investigate a disease outbreak to determine the causative agent as well as its reservoir and route of transmission, so as to recommend ways to minimize the spread. They are concerned with emerging diseases such as that caused by Escherichia coli O157:H7, and also with diseases that have been recognized for centuries, such as tuberculosis and cholera. The studies that epidemiologists conduct are in many ways similar to criminal investigations. After a disease outbreak, investigators conduct a descriptive study to determine the characteristics of the persons involved and the place and the time of the outbreak. This information gives clues as to the possible cause, reservoir of the agent, and transmission of the illness. Once the occurrence of the outbreak has been fully described, an analytical study is done to identify specific conditions, or risk factors, associated with high frequencies of disease. Finally, experimental studies are sometimes done to assess the effectiveness of measures to prevent or treat disease.
The British physician John Snow illustrated the power of a well-designed epidemiological study over a century ago. Years before the relationship between microbes and disease was accepted, he documented that the cholera epidemics plaguing England from 1849 to 1854 were due to contaminated water supplies. He did this by carefully comparing the conditions of households that were affected by cholera and those that were not, eventually determining that the primary difference was their water supply. At one point, he ordered the removal of the handle of a public water pump in the neighborhood of an outbreak; this simple act helped halt an epidemic that in 10 days had killed more than 500 people. ■ cholera, p. 611
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