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Epidemiological Studies Disease Transmission
Field Epidemiology

Epidemiology Defined

An apocryphal story is told around CDC that illustrates the confusion sometimes accompanying the term "epidemiology." It seems that one of our scientists, on first arriving at CDC from a clinical practice, found himself somewhat unsure of what epidemiology was all about, so he sought an answer down the street at Emory University. The first person he asked was a medical student, who told him that epidemiology was "the worst taught course in medical school." The second, a clinical faculty member, told him epidemiology was "the science of making the obvious obscure." Finally, knowing that statistics are important to epidemiology, he asked a statistician, who told him that epidemiology is "the science of long division" and provided him with a summary equation. Giving up on finding a real answer, he returned to CDC. On the way, however, he decided to try one more time. He stopped a native Atlantan who told him that epidemiology was "the study of skin diseases."

A less entertaining, but more conventional, definition of epidemiology is "the study of the distribution and determinants of health-related states in specified populations, and the application of this study to control health problems." A look at the key words will help illuminate the meaning:

  • Study—Epidemiology is the basic science of public health. It's a highly quantitative discipline based on principles of statistics and research methodologies.

  • Distribution—Epidemiologists study the distribution of frequencies and patterns of health events within groups in a population. To do this, they use descriptive epidemiology, which characterizes health events in terms of time, place, and person.

  • Determinants—Epidemiologists also attempt to search for causes or factors that are associated with increased risk or probability of disease. This type of epidemiology, where we move from questions of "who," "what," "where," and "when" and start trying to answer "how" and "why," is referred to as analytical epidemiology.

  • Health-related states—Although infectious diseases were clearly the focus of much of the early epidemiological work, this is no longer true. Epidemiology as it is practiced today is applied to the whole spectrum of health-related events, which includes chronic disease, environmental problems, behavioral problems, and injuries in addition to infectious disease.

  • Populations—One of the most important distinguishing characteristics of epidemiology is that it deals with groups of people rather than with individual patients.

  • Control—Finally, although epidemiology can be used simply as an analytical tool for studying diseases and their determinants, it serves a more active role. Epidemiological data steers public health decision making and aids in developing and evaluating interventions to control and prevent health problems. This is the primary function of applied, or field, epidemiology.

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A comparison between the practice of public health and the more familiar practice of health care helps in describing epidemiology. First, where health care practitioners collect data on an individual patient by taking a medical history and conducting a physical exam, epidemiologists collect data about an entire population through surveillance systems or descriptive epidemiological studies. The health care practitioner uses his or her data to make a differential diagnosis. The epidemiologist's data is used to generate hypotheses about the relationships between exposure and disease. Both disciplines then test the hypotheses, the health care practitioner by conducting additional diagnostic studies or tests, the epidemiologist by conducting analytical studies such as cohort or case-control studies. The final step is to take action. The health care practitioner prescribes medical treatment, and the epidemiologist, some form of community intervention to end the health problem and prevent its recurrence.

One succinct way to sum up the task of epidemiologists is to say that they "count things." Basically, epidemiologists count cases of disease or injury, define the affected population, and then compute rates of disease or injury in that population. Then they compare these rates with those found in other populations and make inferences regarding the patterns of disease to determine whether a problem exists,. For example, in the hepatitis B example earlier, you might ask: Is the rate of disease among people with no know risk factors greater than we would expect? Is the pattern or distribution of the cases suspicious? Once a problem has been identified, the data are used to determine the cause of the health problem; the modes of transmission; any factors that are related to susceptibility, exposure, or risk; and any potential environmental determinants.

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