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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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Epidemiological Studies
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