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Objectives for This Guide

Introduction to Investigating an Outbreak

After studying this guide and answering the questions in the exercises, you will be able to

  • list the reasons that health agencies investigate reported outbreaks

  • list the steps in an outbreak investigation

  • define the terms "cluster," "outbreak," and "epidemic"

  • describe how to determine whether an epidemic actually exists

  • define a "line listing" and describe what it is used for

  • execute the initial steps of an investigation and develop biologically plausible hypotheses

  • draw a traditional epidemic curve

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A Challenge for the Disease Detective

One of the most exciting and challenging tasks facing an epidemiologist—or "disease detective"—working in a public health department is investigating an outbreak. Frequently, the cause and source of the outbreak are unknown. Sometimes large numbers of people are affected. Often, residents are concerned because they fear more people, including themselves, may become ill unless the cause is found quickly. There may be hostility and defensiveness if an individual, product, or company has been accused of being the source of the outbreak. Into this pressure-packed setting comes the epidemiologist from the health department, who must remain calm, professional, and scientifically objective. Fortunately, epidemiology provides the scientific basis, the systematic approach, and the focus on prevention and the population at large that are needed.

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What is an Outbreak?

An outbreak or an epidemic exists when there are more cases of a particular disease than expected in a given area, or among a specific group of people, over a particular period of time. An aggregation of cases in a given area over a particular period, regardless of whether the number of cases is more than expected, is a cluster. In an outbreak or epidemic, we usually presume that the cases are related to one another or that they have a common cause.

Many epidemiologists use the terms "outbreak" and "epidemic" interchangeably; however, some restrict the use of "epidemic" to situations involving large numbers of people over a wide geographic area. The public is more likely to think that "epidemic" implies a crisis situation.

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Uncovering Outbreaks

Health departments learn about most outbreaks in one of two ways. The first and probably most common way is through calls from a doctor, some other health care provider, or a citizen who knows of "several cases." In one such instance in 1989, a nationwide epidemic of a severe illness, eosinophilia-myalgia syndrome (EMS), was first detected when a physician in New Mexico called a consultant in Minnesota and realized that, together, they had seen three patients with highly unusual symptoms. All three patients said they had used the dietary supplement L-tryptophan. The local physician promptly called the New Mexico state health department, setting into motion a chain of actions leading to the discovery of a large nationwide epidemic and the recall of L-tryptophan.

The second means of identifying outbreaks is the routine analysis of public health surveillance data. Through public health surveillance, data on health are systematically collected, analyzed, interpreted, and disseminated on an ongoing basis. This information, which is based on reports sent in by doctors, laboratories, and other sources, allows investigators to track patterns of disease in a community and to determine how to control and prevent it. When, during routine analysis, the data show an increase over the normal background level of reported cases of a particular disease, an outbreak may be indicated.

Back to Top | Next Topic: Why Investigate Outbreaks?

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