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An Introduction to Epidemiology
This
background piece provides several working definitions of epidemiology—the
basic science of public health; an introduction to the different
categories of epidemiology and types of epidemiological studies; and an
overview of the disease transmission cycle. First, to set the stage,
consider the three incidents that follow, stepping into the shoes of the
public health officer who received the initial report and asking yourself
the question, "What do I do now?" Some of these examples made
national news and may be familiar to you.
First Incident
In March 1985, a nurse epidemiologist in a county health department noted,
while reviewing surveillance data, three cases in a single month of
hepatitis B of unusual origin. Hepatitis B, or serum hepatitis, is
transmitted through sexual contact and by exposure to infected bodily
fluids, but these three patients did not seem to have the usual risk
factors. All three people did, however, indicate having received
injections at the same health care facility.
The nurse's
immediate questions were: Is this a coincidence? Did these three cases
occur by chance or is there a link? In this instance, the nurse decided to
pursue an investigation.
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Second Incident
At 8:30 in the morning on August 2, 1976, Dr. Robert B. Craven of CDC's
Viral Diseases Division received a call from a nurse at a Veterans'
Hospital in Philadelphia, Pennsylvania. The nurse reported two cases of
severe respiratory illness, one of which had been fatal. Both people had
attended the annual American Legion Convention held July 21-24 . By the
evening of August 2, 71 more of the people attending the convention had
the same illness, with symptoms of acute onset of fever, chills, headache,
malaise, dry cough, and myalgia. Further conversations with local and
state public health officials revealed that between July 26 and August 2,
18 conventioneers had died. Deaths were due primarily to pneumonia.
An intense
investigation began immediately. The incident became known as the first
outbreak of Legionnaires' disease and led to the discovery of the
gram-negative pathogen, Legionnella pneumophila.
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Third Incident
On October 30, 1989, a New Mexico physician notified the state's health
department of three patients with marked peripheral eosinophilia and
severe myalgia. All three patients had been taking oral preparations of L-tryptophan,
a nonprescription drug sold as a dietary supplement in health food stores.
Despite extensive clinical evaluation and testing, the illness could not
be identified.
An
investigation followed and resulted in the characterization of
eosinophilia-myalgia syndrome, EMS. The investigation implicated a vehicle
for exposure—L-tryptophan dietary supplements—before a suspected agent
was identified, and the product was taken off the market. Eventually, the
problem was traced to a contaminant that had been introduced by changes in
the production process at a single manufacturing facility.
These three
examples illustrate some of the key reasons for needing applied, or field,
epidemiology:
- They
were unexpected.
- They
demanded a response.
- The
investigators had to go out into the field to solve the problem.
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Next Topic: Epidemiology Defined
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