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Lesson 6: Investigating an Outbreak

Section 1: Introduction to Investigating an Outbreak

Uncovering outbreaks

To uncover outbreaks:

  • Review routinely collected surveillance data
  • Astutely observe single events or clusters by clinicians, infection control practitioners, or laboratorians
  • Review reports by one or more patients or members of the public

Outbreaks of disease — the occurrence of more cases than expected — occur frequently. Each day, health departments learn about cases or outbreaks that require investigation. While CDC recorded over 500 outbreaks of foodborne illness alone each year during the 1990s,(1) recognized outbreaks of respiratory and other diseases are also common, and many more outbreaks may go undetected.

So how are outbreaks uncovered? One way is to analyze surveillance data — reports of cases of communicable diseases that are routinely sent by laboratories and healthcare providers to health departments (see Lesson 5). Some health departments regularly review exposure information from individual case reports to look for common factors. For example, health department staff in Oregon uncovered an outbreak of E. coli O157:H7 in 1997 by noticing that three patients with the infection all had reported drinking raw milk.(2) Alternatively, outbreaks may be detected when health department staff conduct regular, timely analysis of surveillance data that reveals an increase in reported cases or an unusual clustering of cases by time and place. For example, by analyzing data from four different syndromic surveillance systems, health department staff in New York City noted a consistent increase in gastroenteritis in the days following a prolonged blackout in August 2003.(3) Investigation indicated that the increase in gastroenteritis was probably attributable to the consumption of meat that had spoiled during the power failure.

Review of surveillance data to detect outbreaks is not limited to health departments. Many hospital infection control practitioners review microbiologic isolates from patients by organism and ward each week to detect an increase in the number of, say, surgical wound infections or nosocomial (hospital-acquired) cases of legionellosis. In the same way, staff at CDC regularly review laboratory patterns of organisms and are able to detect clusters of illness caused by the same organism, even if the victims are geographically scattered.(4)

Nonetheless, most outbreaks come to the attention of health authorities because an alert clinician is concerned enough to call the health department. The emergence of West Nile virus infection in North America in 1999 was uncovered only after the New York City health department responded to a call from a physician who had recently seen two patients with encephalitis.(5) Similarly, a single case of inhalational anthrax of suspicious origin in Florida in 2001 resulted in a massive investigation involving multiple government agencies, but it all started with an astute diagnosis and prompt report to the health department by a physician.(6)

Epidemic: the occurrence of more cases of disease than expected in a given area or among a specific group of people over a particular period of time. Usually, the cases are presumed to have a common cause or to be related to one another in some way

Outbreak: epidemic limited to localized increase in the incidence of disease

Cluster: aggregation of cases in a given area over a particular period without regard to whether the number of cases is more than expected

Another reporting source for apparent clusters of both infectious and noninfectious disease is patients or other members of the community. For example, an individual may call the health department and report that she and some friends came down with severe gastroenteritis after attending a banquet a night or two earlier. Similarly, a local citizen may call about several cases of cancer diagnosed among his neighbors and express concern that these are more than coincidental. Most health departments have routine procedures for handling calls from the public regarding potential communicable disease outbreaks, and some states have guidelines for how to respond to noninfectious disease cluster reports.(79)

Deciding whether to investigate a possible outbreak

Different health departments respond to these reports in different ways. The decisions regarding whether and how extensively to investigate a potential outbreak depend on a variety of factors. These usually include some factors related to the health problem, some related to the health department, and some related to external concerns. Factors related to the problem itself include the severity of the illness, the number of cases, the source, mode or ease of transmission, and the availability of prevention and control measures. Most local health departments are more likely to investigate an apparent outbreak when the number of affected (or exposed) persons is large, when the disease is severe (serious illness with high risk of hospitalization, complications, or death), when effective control measures exist, and when the outbreak has the potential to affect others unless prompt control measures are taken. For example, a single case of gastroenteritis is unlikely to prompt a field investigation, but a cluster of cases may. On the other hand, even a single case of botulism is likely to be investigated immediately to identify and eliminate the source, because it is both potentially fatal and preventable, and the source can usually be identified. At the state or national level, the unusual presentation of disease may spur an investigation. Occurrence of a new or rare disease or a change in the pattern of disease in an area is more likely to prompt an investigation than occurrence of a common disease with well-established transmission patterns and control measures.

However, field investigations place a burden on a health department, so the decision also hinges on the availability of staff and resources, and competing priorities. In addition, some health departments have a practice of aggressively investigating outbreaks and hence have experience in doing so, while other health departments may lack such experience.

Regardless, field investigations are usually justified for one or more of the following reasons:

  • Control or prevention of the health problem
  • Opportunity to learn (research opportunity)
  • Public, political, or legal concerns
  • Public health program considerations
  • Training

Each of these reasons is discussed in more detail below.

Control and prevention

The most important public health reasons for investigating an outbreak are to help guide disease prevention and control strategies. These disease control efforts depend on several factors, including knowledge of the agent, the natural course of the outbreak, the usual transmission mechanism of the disease, and available control measures. For example, if a health department learns of an outbreak of hepatitis A (known agent) in which one of the victims is a restaurant cook, the department can offer immune globulin to the restaurant patrons to prevent a second wave of cases (control measure), but only if they are within 14 days of exposure (timing). On the other hand, if an outbreak appears to be almost over, the health agency may not need to implement control measures, but may be interested in identifying factors that contributed to the outbreak in order to develop strategies to prevent similar outbreaks in the future. For that outbreak of hepatitis A, investigators may find that the poor personal hygiene that led to the outbreak was the result of lack of soap or water in the workplace washroom, which could be addressed in public health messages to other worksites.

The balance between control measures and further investigation depends on how much is known about the cause, the source, and the mode of transmission of the agent.(10) Table 6.1 illustrates how public health emphasis on investigation versus control is influenced by these factors. In particular, if the source and/or mode of transmission are known, then control measures that target the source or interrupt transmission can be implemented. If the source and/or mode of transmission are not known, then you can't know what control measures to implement, so investigation takes priority.

Table 6.1 Relative Priority of Investigative and Control Efforts During an Outbreak, Based on Knowledge of the Source, Mode of Transmission, and Causative Agent

Source/Mode of Transmission
(How people are getting exposed to the agent)
Causative AgentKnownInvestigation +
Control +++
Investigation +++
Control +
UnknownInvestigation +++
Control +++
Investigation +++
Control +

+++ = highest priority
+ = lowest priority

Source: Goodman RA, Buehler JW, Koplan JP. The epidemiologic field investigation: science and judgment in public health practice. Am J Epidemiol 1990;132:9–16.

Opportunity to learn (research opportunity)

Another important objective of many outbreak investigations is to advance research. For most public health problems, health officials cannot conduct randomized trials. We cannot randomize who eats the undercooked hamburger or sits near the ice resurfacing machine that emits carbon monoxide, nor should we randomize who receives preventive health benefits (e.g., mammogram screening). However, we can take advantage of what has already happened and learn from it. Some view an outbreak as an experiment of nature waiting to be analyzed and exploited. For a newly recognized disease, field investigation provides an opportunity to characterize the natural history — including agent, mode of transmission, and incubation period — and the clinical spectrum of disease. Investigators also attempt to characterize the populations at greatest risk and to identify specific risk factors. Acquiring such information was an important motivation for investigators studying such newly recognized diseases as Legionnaires' disease in Philadelphia in 1976, AIDS in the early 1980s, hantavirus in 1993, severe acute respiratory syndrome (SARS) in 2003, and avian flu in 2005.

Even for diseases that are well characterized, an outbreak may provide opportunities to gain additional knowledge by assessing the impact of control measures and the usefulness of new epidemiology and laboratory techniques. For example, outbreaks of varicella (chickenpox) in highly immunized communities allowed investigators to determine effectiveness of the new vaccine and immunization recommendations.(11, 12) An outbreak of giardiasis provided the opportunity to study the appropriateness of a clinical case definition,(13) while an outbreak of rotavirus was used to study the performance of a novel diagnostic method.(14) With increased access to the Internet and e-mail in the 1990s, outbreak investigations were used to evaluate whether potential controls would respond to e-mail solicitations to participate.(1517)

Public, political, or legal concerns

Public, political, or legal concerns can be the driving force behind the decision to conduct an investigation. A cluster of cancer cases in a neighborhood may prompt concerned residents to advocate for an investigation. Sometimes the public is concerned that the disease cluster is the result of an environmental exposure such as toxic waste. Investigations of such clusters almost never identify a causal link between exposure and disease.(18,19) Nevertheless, many health departments have learned that they must be "responsibly responsive" to public concerns, even if they think that an epidemiologic link is unlikely.(7,8,20) Similarly, the public may fear that an outbreak is the result of an intentional criminal or bioterrorist act. The health department may be able to allay those fears by documenting that the outbreak was the result of an inadvertent or naturally occurring exposure.

Some investigations are conducted because they are required by law. For example, CDC's National Institute for Occupational Safety and Health (NIOSH) is required to evaluate the risks to health and safety in a workplace if requested to do so by a union, three or more workers, or an employer.(21)

Program considerations

Many health departments run programs to control and prevent communicable diseases such as influenza, tuberculosis, vaccine-preventable diseases, and sexually transmitted diseases. An outbreak of a disease targeted by a public health program may reveal a weakness in that program and an opportunity to change or strengthen program efforts. Investigating the outbreak's causes may identify populations that have been overlooked, failures in intervention strategies, or changes in the agent. Using the outbreak to evaluate program effectiveness can help program directors improve future directions and strategies.


Investigating an outbreak requires a combination of diplomacy, logical thinking, problem-solving ability, quantitative skills, epidemiologic know-how, and judgment. These skills improve with practice and experience. Thus, many investigative teams pair a seasoned epidemiologist with an epidemiologist-in-training. The latter gains valuable on-the-job training and experience while providing assistance in the investigation and control of the outbreak.

Pencil graphic Exercise 6.1

During the previous year, nine residents of a community died from cervical cancer. List at least 4 reasons that might justify an investigation.

Check your answer.

References (This Section)

  1. Olsen SJ, MacKinon LC, Goulding JS, Bean NH, Slutsker L. Surveillance for foodborne disease outbreaks — United States, 1993–1997. In: Surveillance Summaries, March 27, 2000. MMWR 2000; 49(No. SS-1):1–59.
  2. Keene WE, Hedberg K, Herriott DE, Hancock DD, McKay R, Barrett T, Fleming D. A prolonged outbreak of Escherichia coli O157:H7 infections caused by commercially distributed raw milk. J Infect Dis 1997;176:815–8.
  3. Marx M. Diarrheal illness detected through syndromic surveillance after a massive blackout, New York City. Presented at 2003 National Syndromic Surveillance Conference.
  4. Swaminathan B, Barrett TJ, Hunter SB, Tauxe RV. PulseNet: the molecular subtyping network for foodborne bacterial disease surveillance, United States. Emerg Infect Dis 2001; 7:382–9.
  5. Preston R. West Nile mystery. The New Yorker, October 18–25,1999;90–107.
  6. Bush LM, Abrams BH, Beall A, Johnson CC. Index case of fatal inhalational anthrax due to bioterrorism in the United States, N Engl J Med 2001;345:1607–10.
  7. Bender AP, Williams AN, Johnson RA, Jagger HG. Appropriate public health responses to clusters: the art of being responsibly responsive. Am J Epidemiol 1990;132:S48–S52.
  8. Fiore BJ, Hanrahan LP, Anderson HA. State health department response to disease cluster reports: a protocol for investigation. Am J Epidemiol 1990;132:S14–22.
  9. Washington State Department of Health. Guidelines for investigating clusters of chronic disease and adverse birth outcomes [monograph on the Internet]. Olympia, Washington; 2001.
  10. Goodman RA, Buehler JW, Koplan JP. The epidemiologic field investigation: science and judgment in public health practice. Am J Epidemiol 1990;132:9–16.
  11. Galil K, Lee B, Strine T, Carraher C, Baughman AL, Eaton M, et al. Outbreak of varicella at a day-care center despite vaccination. New Engl J Med 2002;347:1909–15.
  12. Tugwell BD, Lee LE, Gillette H, Lorber EM, Hedberg K, Cieslak PR. Chickenpox outbreak in a highly vaccinated school population. Pediatrics. 2004;113:455–9.
  13. Hopkins RS, Juranek DD. Acute giardiasis: an improved clinical case definition for epidemiologic studies. Am J Epidemiol 1991;133:402–7.
  14. Fischer TK, Gentsch J, Ashley D, et al. Evaluation and utility of a novel diagnostic method in the investigation of an unusual outbreak of rotavirus diarrhea among children — Jamaica, 2003. Presented at: 53rd Annual EIS Conference, CDC, Atlanta, Georgia, April 19–23, 2004.
  15. Raupach JC, Hundy RL. An outbreak of Campylobacter jejuni infection among conference delegates. Commun Dis Intell 2003;27:380–3.
  16. Kuusi M, Nuorti JP, Maunula L, Miettinen I, Pesonen H, von Bonsdorff C-H. Internet use and epidemiologic investigation of gastroenteritis outbreak. Emerg Infect Dis 2004;10:447–50.
  17. Pryor JH, Martin MT, Whitney CG, Turco JH, Baumgartner YY, Zegans ME. Rapid response to a conjunctivitis outbreak: the use of technology to leverage information. J Am Coll Health 2002;50:267–71.
  18. Caldwell GG. Twenty-two years of cancer cluster investigations at the Centers for Disease Control. Am J Epidemiol 1990;132:S43–S47.
  19. Schulte PA, Ehrenberg RL, Singal M. Investigation of occupational cancer clusters: theory and practice. Am J Public Health 1987;77:52–6.
  20. Cartwright RA. Cluster investigations: are they worth it? Med J Aust 1999;171:p. 172.
  21. Centers for Disease Control and Prevention. NIOSH health hazard evaluation program. Cincinnati, Ohio: Department of Health and Human Services (NIOSH) Publication No. 2000-132: p. 3