Investigate an Outbreak

Defining an Outbreak of Unexplained Respiratory Illness

A cluster is “an aggregation of cases grouped in place and time that are suspected to be greater than the number expected, even though the expected number may not be known”.  An outbreak  is when there is more disease cases than what is usually expected:

  • For a given time (e.g., within 2 weeks)
  • Within a specific location (e.g., linked by institution, affiliation, exposure, small geographic area)
  • For a target population (e.g., students, long-term care residents)

As additional information about a particular outbreak becomes available, it may make sense to adjust the outbreak definition. Consider incorporating:

  • Predominant clinical presentations (e.g., pneumonia, pharyngitis)
  • Severity of illness (e.g., hospitalizations, deaths)
  • Demographic profiles
  • Illness onset dates of the cases (see Line List Template pdf icon[4 pages])
  • A comparison to the historical or baseline rates for similar disease in the community

In certain situations, public health officials may need to evaluate a single case of unexplained respiratory disease as a possible outbreak. Examples include diseases with the potential need for immediate public health intervention (e.g., suspect pulmonary anthrax, plague, SARS, MERS, hantavirus pulmonary syndrome).

Which Outbreaks Should Be Investigated?

For each respiratory disease outbreak, public health officials determine the appropriate level of public health response. They must consider many factors in making the decision, such as availability of resources and competing agency priorities. However, several characteristics of respiratory outbreaks typically warrant further investigation and an urgent response. The characteristics below, while not comprehensive or definitive, can help determine which outbreaks merit further investigation:

  • Outbreaks of unknown etiology
  • Outbreaks associated with severe disease manifestations, such as need for hospitalization or death
  • Outbreaks that may be useful to answer epidemiologic, laboratory, or infection control questions
  • Outbreaks of possible vaccine-preventable diseases
  • Outbreaks associated with institutional settings or with a likely (controllable) environmental source
  • Outbreaks of respiratory infection potentially caused by a bioterrorism agent
  • Outbreaks among a vulnerable population
  • Outbreaks that have generated excessive public anxiety
  • Outbreaks that are either very large or rapidly progressing

See Request CDC Assistance for guidance on when and how to reach out to CDC for help investigating an unexplained respiratory disease outbreak.

Components for Epidemiological Investigation of Outbreaks

Below are some of the common components of an epidemiological outbreak investigation. The circumstances in any given outbreak may require additional steps.

  • Start to collect information
    Begin collecting available epidemiologic, clinical, and laboratory information to help assess the situation. See Respiratory Outbreak Survey pdf icon[1 page] for an example on the types of information you might want to gather. The following tools will help guide the next steps:
  • Develop public health response to outbreak
    The initial information gathered may lead to additional questions in order to gauge the degree of response. The following steps may help inform an appropriate response:
    • Determine the number of cases and severity of disease
    • Assess the need and potential for interventions (e.g., cohorting, quarantine, vaccination, use of prophylaxis, elimination of potential disease source)
    • Consider the likelihood of natural versus intentional source of infection
    • Evaluate the level of public health, clinician, or community concern
  • Report and respond
    Coordinate the public health response by:
    • Discussing with staff in your program (e.g., laboratory, epidemiology, environmental, veterinary, other personnel as appropriate)
    • Notifying appropriate local or state public health officials
    • Notifying CDC, if indicated, of the outbreak by e-mailing URDO (M-F; 8 am–4 pm EST) or calling the emergency, after-hours hotline at 770-488-7100.

Data Collection Forms

Data Collection Forms
Public health officials may wish to gather additional information about possible exposures. These forms can assist in the interview and investigation of single cases of disease. Adapt these forms to collect information relevant to identifying etiologies of respiratory outbreaks. Do not use these forms to report a disease outbreak to CDC. For reporting assistance, please refer to your State Health Department.


Outbreaks of acute respiratory illness are common and can occur in many settings (e.g., communities, nursing homes, military barracks). Specific therapies, infection control practices, and other preventive measures may be necessary to control outbreaks. Consequently, investigation into the clinical and epidemiologic features, as well as the etiology of outbreaks is particularly important to public health. Investigating severe unexplained respiratory outbreaks may be of particular importance to rule out emerging health threats.

Predominant clinical respiratory syndromes include outbreaks of

  • Prolonged paroxysmal cough
  • Bronchitis
  • Pneumonia
  • Influenza-like illness
  • Acute respiratory distress syndrome or rapidly progressive pneumonia

Despite continual efforts, the etiology of many acute respiratory illnesses may remain unexplained. Investigating unexplained respiratory outbreaks may identify infectious agents that require unique strategies for control and prevention (e.g., hantavirus pulmonary syndrome, SARS); however, outbreak investigations for previously identified etiologies are equally important. This process can be enhanced through coordinated efforts that:

  1. Alert clinicians to identify and report cases and clusters of unexplained infectious diseases
  2. Prepare health departments to investigate these clusters

Guide laboratory scientists to develop and apply both traditional and novel diagnostic approaches to define the organism-specific etiology of the outbreak