Epidemiology

The Development of Epidemiology

The work of John Snow in 1854 shaped the development of epidemiology. Working to understand an outbreak of cholera in the Soho district of London, Snow mapped the geographical location of victims and investigated what they may have had in common. One factor was that they were drinking water that originated in a common and contaminated pump. Snow [Page 615]closed the pump and was able to contribute toward stopping the epidemic. Importantly, he achieved this without knowledge of the bacterial mechanisms underlying the disease, and his methods influenced subsequent epidemiological research. This marked the first era of epidemiology, wherein sanitary conditions were the primary focus of research. This was followed by an infectious disease era (late 19th century to mid20th century) and the development of germ theory. After World War II and coinciding with the growing prevalence of chronic diseases, epidemiological research increasingly focused on individual-level risk factors and “healthy lifestyles.” This era was marked by an increasing acceptance of a “web of causality” model, in which no single causal agent would be expected to explain patterns of disease. In contrast, epidemiological research often operated within a “black box” paradigm, where associations could be theorized and investigated as causal without an understanding of biological mechanisms. This was a strength of epidemiology, for it could expand its research into areas not yet studied by other branches of science. However, it was also an important weakness, and criticisms have been made that associations without plausible underlying biological mechanisms do little to advance knowledge of public health concerns and may lead to false-positive (spurious) findings.

The Tools of Epidemiology

In practice, epidemiology is a very diverse field, and it is practiced by social scientists, physicians, demographers, biologists, and a wide range of other specialists in both the social and natural sciences. The central concept in epidemiology is a case, which can denote an episode of a disorder or illness, or an event, such as death. Epidemiologists study rates of incidence, or the number of new cases during a specific period of time, as well as rates of prevalence, or the total number of cases in a given period of time. Prevalence is often reported as a measure of point prevalence (at a specific point in time, such as a day), period prevalence (during a specific period of time, such as a year), and lifetime prevalence (reflecting the number of people expected to have a particular disorder at least once in their lifetime). Central to the work of epidemiologists is also the notion of a rate. This can be a crude rate, reflecting the number of cases per population, for example, infant mortality per 1,000 live births. Adjusted rates are also used to examine the effects of specific variables or social characteristics, for example, age-adjusted mortality rates.

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