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
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
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 ...