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The first example of the use of epidemiology to study and control a disease occurred in London between 1853 and 1854. It was conducted by a British physician, John Snow, who is known as the father of modern epidemiology. Snow was concerned about a cholera epidemic that had struck London in 1848. He noticed that death rates were especially high in parts of the city with water supplied by two private companies, both of which drew water from the Thames River at a point heavily polluted with sewage. Between 1849 and 1854, the Lambeth Company changed its source to an area of the Thames that was free of pollution from London's sewers. Snow noticed that the number of cholera deaths declined in the section of London supplied by the Lambeth Company, while there was no change in the sections supplied by the Southwark and Vauxhall Company. He formulated the hypothesis that cholera was spread by polluted drinking water. In 1853, there was a severe outbreak of cholera concentrated in the Broad Street area of London, in which some houses were supplied by one water company and some by the other. This provided an opportunity for Snow to test his hypothesis in a kind of natural experiment. Snow went to each house in which someone had died of cholera between August 1853 and January 1854 to determine which company supplied the water. When he tabulated the results, he found that the rate of cholera deaths was thus 8.5 times higher in houses supplied by the Southwark and Vauxhall Company than those supplied by the Lambeth Company. This was convincing evidence that deaths from cholera were linked with the source of water.
In 1948, an epidemiologic study was launched in Framingham, Massachusetts, to investigate factors that might be causing the problem. It was the first major epidemiologic study of a chronic disease. More than half of the middle-aged population of the town, more than 5000 health people, were examined, and data were recorded on their weight, blood pressure, smoking habits, the results of various blood tests, and other characteristics. Two years later, the same people were examined again, and these tests have been and continue to be repeated every 2 years for the rest of their lives. As early as 10 years later, the Framingham Heart Study had revealed a great deal about how to predict which of their subjects were likely to develop heart disease. The study identified three major risk factors: high blood pressure, high blood cholesterol, and smoking. As a result of these findings, concepts of normal blood pressure and cholesterol levels changed significantly. Doctors had previously believed that blood pressure naturally increased as people aged and that the increase was normal and healthy. Remarkably, the Framingham findings had a major impact on the course of the heart disease epidemic. Publicity on the information gained by the study, confirmed and supported by other studies, persuaded some people to change their behavior and formed the basis of public health programs to encourage others to do the same. By the 1970s, it was clear that death rates from heart disease were falling in the US. These beneficial trends have continued.