2 Early studies in epidemiology
The following three examples serve to illustrate, albeit very briefly, the origins of modern epidemiology.
In the mid-nineteenth century, Dr John Snow suspected that drinking water might have caused the spread of cholera in Soho, London. He plotted all the cholera cases in Soho on a local map and noted that they clustered around Broad Street, which had a water pump. He found that people who had drunk water provided by a particular water company were more likely to have developed cholera than those who had not. Snow’s interest was not so much in treating the individuals with cholera as in the patterns of where the victims lived and where they obtained their water supply, so that he could prevent further spread of the disease. Thus, the link between the water and cholera was established before it was understood precisely what was in the water that caused the illness. Hence, the epidemiological explanation preceded the biological explanation and was able to halt the rapid spread of the disease.
There are two twentieth-century parallels to this nineteenth-century example: the link between smoking and increased incidence of lung cancer (Doll and Hill, 1950) and AIDS, for which understanding of the biology of the disease is far behind epidemiological explanations of how the infection spreads. Epidemiology can, thus, help identify populations or certain groups with an above-average death rate, or high disease rates, and that information is essential to all those involved in working towards equality in health.
There are two broad types of epidemiological inquiry: one is descriptive, the other analytic. Although they are interdependent, they use different methods. Before exploring the scope of these two types of epidemiology, the basic levels of measurement are explained. These are measures of deaths in a population (mortality) and measures of disease in a population (morbidity).