Chapter 5

• A disease outbreak is an epidemic that occurs suddenly and within a relatively confined geographic area.
• The occurrence of a disease outbreak requires a pathogen in sufficient quantities, a mode of transmission, and a pool of susceptible persons.
• The two primary modes of transmission of pathogens in disease outbreaks are person-to-person spread and common source of exposure.
• Criteria for determining whether a disease outbreak should be investigated include, among others: the number and severity of persons affected, uncertainty about cause, and level of public concern.
• The attack rate is a measure of the number of persons affected by the disease outbreak among persons at risk.
• Food-borne pathogens can give rise to disease outbreaks through a common source exposure.
• Emerging infectious disease refers to an infection that has newly appeared in a population, or has existed but is rapidly increasing in incidence or geographic spread.
• Certain emerging pathogens, such as severe acute respiratory syndrome (SARS)–related coronavirus, West Nile virus, and Ebola virus are responsible for highly publicized recent outbreaks of disease with high case-fatalities.
• In 2001, the threat of bioterrorism was realized when Bacillus anthracis was mailed to at least five locations, resulting in 22 cases of anthrax and five deaths.

A 23-year-old male student presented at 10:30 pm on January 17 at the college infirmary complaining of a sudden onset of abdominal cramping, nausea, and diarrhea. Although the patient was not in severe distress and had no fever or vomiting, he was weak. A number of other students, all with the same symptoms, visited the college infirmary over the next 20 hours. All patients were treated with bed rest and fluid replacement therapy. They recovered fully within 24 hours of the onset of illness.

The concept of an epidemic as a dramatic rise in the occurrence of a disease was introduced in Chapter 3: Patterns of Occurrence. When an epidemic occurs suddenly and in a relatively limited geographic area, it is described as a disease outbreak. The emergence of a disease outbreak requires immediate action to determine the origin of the problem, and ultimately to prevent other persons from becoming affected.

In many outbreak situations, distinctive clinical features of the affected individuals may suggest the underlying cause (sometimes termed pathogen). A working hypothesis can lead to prompt identification of the causal agent and implementation of control measures. Ideally, the choice of control strategy is predicated on knowledge of the source of the causal agent and how it is spread.

In other circumstances, however, the clinical features of affected individuals do not suggest a particular pathogen. An urgent response is required, although the investigator does not yet have a specific working hypothesis about the cause. Consequently, the first phase of investigation involves the collection of basic descriptive information to better characterize the illness and its pattern of occurrence. With this background descriptive data in hand, investigators can generate hypotheses and design specific ...

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