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Investigating an outbreak/epidemic is a set of procedures used to identify the cause responsible for the disease, the people affected, the circumstances and mode of spread of the disease, and other relevant factors involved in propagating the epidemic, and to take effective actions to contain and prevent the spread of the disease. Conducting an outbreak investigation requires planning and strategy to answer these key questions.

A disease outbreak/epidemic is the occurrence of cases of disease in excess of what would normally be expected in a defined community, geographical area or season.

An outbreak/epidemic may occur in a restricted geographical area, or may extend over, several neighboring areas, whole country or several countries. It may last for a few days or weeks, or for several years.

Epidemic: It is 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. Epidemic is usually applied to situations involving larger numbers of people over a wide geographic area. 

Outbreak: Epidemic limited to localized increase in the incidence of disease (such as village, town, or closed institution) can be termed as outbreak.


A single case of a communicable disease long absent from a population, or caused by an agent not previously recognized in that community or area, or the emergence of a previously unknown disease, may also constitute an outbreak and should be reported and investigated.

Outbreaks may be detected while analyzing surveillance data, reporting by health facility/ medical practioners, affected persons/groups, local leaders, or by keeping rumour registry or through media.

Deciding whether to investigate a possible outbreak

The decision regarding whether and how extensively to investigate a potential outbreak depends on a variety of factors. These factors may be related to the health problem, related to the health department, and some factors may have 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.




Field investigations are usually justified for one or more of the following reasons:

  • Control or prevention of the health problem: The most important public health reason for investigating an outbreak is to help guide disease prevention and control strategies.
  • Opportunity to learn (research opportunity): Another important objective of many outbreak investigations is to advance research.
  • Public health program considerations: Sometimes the outbreak investigation is used to evaluate program effectiveness.
  • Training: Investigating an outbreak for training the concerned team members.

Once the decision to conduct a field investigation of an acute outbreak has been made, a systematic approach ensures that the investigation proceeds without missing important steps along the way.

Steps of an Outbreak Investigation

Step 1: Prepare for field work

Step 2: Establish the existence of an outbreak

Step 3: Verify the diagnosis

Step 4: Define and identify cases

Step 5: Perform descriptive epidemiology

Step 6: Develop hypotheses

Step 7: Evaluate hypotheses/ perform additional studies as necessary

Step 8: Implement control and prevention measures

Step 9: Communicate findings

Step 10: Follow up recommendations

1: Prepare for field work

Investigator should be well prepared to conduct a field investigation before leaving for the field regardless of when the decision to conduct a field investigation is made, because sometimes preparing for field work is not the first step.

The preparations can be grouped into two broad categories:

(a) scientific and investigative issues, and (b) management and operational issues

Scientific and investigative issues: Before leaving for a field investigation, investigator must have appropriate scientific knowledge about the disease and about field investigation. Investigator should have a plan of action, necessary supplies and equipment, the proper laboratory material and should know the proper collection, storage, and transportation techniques for collected material. Some outbreak investigations require no special protective equipment; while investigation for some diseases requires personal protective equipment such as masks, gowns, and gloves to protect investigating team.

Management and operational issues: An investigator must be a good manager also, because most investigations are conducted by a team rather than just one individual. The team members must be selected before departure and they should know their expected roles and responsibilities in the field. Operational and logistical details must be clear before proceeding to field.

Step 2: Establish the existence of an outbreak


The expected number is usually the number from the previous few weeks or months, or from a comparable period during the previous few years and may be based on locally available data such as information from outpatient records of health facility, integrated disease surveillance programme, hospital admission/discharge records, mortality statistics, or data from neighouring areas, community survey.

To establish the existence of an outbreak, the observed number of cases is compared with the expected number for a certain place or time period.

Even if the current number of reported cases exceeds the expected number, the excess may not necessarily indicate an outbreak. Therefore, look for the various factors that may increase the number of cases such as:

  • Seasonal variation: There is seasonal increase in some diseases-diarrhea.
  • Notification variation: Reporting of cases may rise because of change in local reporting procedure, changes in the case definition, increased interest because of local or national awareness.
  • Sudden change in population size such as resort areas, college towns, and migrant farming areas.
  • Diagnostic variation (new technique)-Change or improvements in diagnostic procedures may increase the cases.
  • Diagnostic mistake (false epidemic)- Some apparent increases may be the result of misdiagnosis or laboratory error.

Step 3: Verify the diagnosis/ ensure proper diagnosis of reported cases

Verifying the diagnosis is closely linked to verifying the existence of an outbreak. This is important: (a) to ensure that the disease has been properly identified, since control measures are often disease-specific; and (b) to rule out laboratory error as the basis for the increase in reported cases.

For verification of diagnosis:

  • Investigator should talk to health workers first, then review the clinical findings and laboratory results.
  • If laboratory diagnosis in not made or there is any doubt in diagnosis, team member may collect a sufficient number of appropriate specimens for further confirmation of disease.
  • It is useful to visit one or more patients with the disease, talking directly with some patients can give a better understanding of the clinical feature.
  • Summarize the clinical findings with frequency distribution (number of cases).

Step 4: Define and identify cases

A case definition is constructed for the identification of cases. It is a standard set of criteria for deciding whether an individual should be classified as having the health condition of interest. A case definition includes clinical criteria (signs and symptoms) and restrictions by time, place, and person. The criteria must be applied consistently to all persons under investigation.

Investigators often create different categories of a case definition, such as confirmed, probable, and possible or suspect cases. To be classified as confirmed, a case usually must have laboratory verification. A case classified as probable usually has typical clinical features of the disease without laboratory confirmation. A case classified as possible usually has fewer of the typical clinical features. Exposure or risk factor should not be included in the case definition.

Step 5: Perform descriptive epidemiology (Find cases systematically and record information)

In an outbreak situation, cases that prompt the concern are often only a small and do not represent the total number of cases. Therefore, with the help of public health workers, investigator must look for additional cases to determine the true geographic extent of the problem and the populations affected by it. All the cases should be counted as reported by either passive surveillance (patient coming directly to health facility) or reported by active surveillance (health worker searching for the cases).

Line listing: Information about each person who is listed with concerned disease during an outbreak is created in a tabular form on paper (hard copy) or on a computer (electronic version) is termed as line listing.

A line listing should be prepared consisting of following details:

  • Identifying information: Name, address, and telephone number
  • Demographic information: Age, sex, race, occupation-the information related to the person characteristics is needed to characterize the populations at risk.
  • Clinical information includes signs and symptoms, date of onset of illness, other supplementary clinical information such as duration of illness and outcome of disease whether hospitalized, improved or death occurred.
  • Risk factor information: This information must be tailored to the specific disease in question. For example, since food and water are common vehicles for hepatitis A but not hepatitis B, exposure to food and water sources must be ascertained in an outbreak of the former but not the latter.
  • Reporter information: The case report must include the source of the report, usually a medical officer, or physician, clinic, hospital, or laboratory. Investigator sometimes needs to contact the reporter, either to seek additional clinical information or report back the results of the investigation.

The next step after identifying and gathering basic information about the persons with the disease is to systematically describe some of the key characteristics of those persons in terms of time, place, and person. This is called descriptive epidemiology.

Time: Traditionally, a special type of curve or histogram is used to depict the time course of an epidemic. This graph is called an epidemic curve (epi curve). It provides a simple visual display of the outbreak's magnitude and time trend.

To draw an epidemic curve*, investigator must know the time of onset of illness for each case. For some diseases, date of onset is sufficient. For other diseases, particularly those with a relatively short incubation period (the period between exposure to an infection and the appearance of the initial symptoms), hour of onset may be more suitable.

Epidemic curve gives various information about the outbreak:

  • Magnitude of the outbreak: The epi curve shows the magnitude of the epidemic over time.
  • The shape of the epidemic curve may provide clues about the pattern of spread of outbreak in the population, e.g., point source, intermittent source or propagated outbreak.
  • Outbreak time trends: The curve shows the different stage of epidemic- still on the upswing, or on the down slope, or after the epidemic when it has ended.
  • The curve can be used for evaluation, answering questions like: How long did it take for the health department to identify a problem? Are intervention measures working?
  • Outbreak outliers: Cases that don't fit into the body of the curve may provide important clues.
  • Period of exposure/incubation period for the outbreak: If the timing of the presumed exposure is known, epi curves can be used to estimate the incubation period of the disease, and this may facilitate identification of the causative agent.
  • If the disease and its incubation period are known, the epi curve can be used to detect a probable time of exposure and help develop a questionnaire focused on that time period.

Place: Assessment of an outbreak by place provides information on the geographic extent of a problem, and also demonstrates clusters or patterns that provide important etiologic clues. A spot map is a simple and useful technique for illustrating where cases live, work, or may have been exposed.

Step 6: Develop hypotheses

The descriptive epidemiology may provide useful clues that can be turned into hypotheses. In an outbreak context, hypotheses are generated in a variety of ways. First, consider what is known about the disease itself: What is the agent's usual reservoir? How is it usually transmitted? What vehicles are commonly implicated? What are the known risk factors?

Step 7: Evaluate hypotheses

After a hypothesis that might explain an outbreak has been developed, the next step is to evaluate the plausibility of that hypothesis. Typically, hypotheses in a field investigation are evaluated using a combination of environmental evidence, laboratory science, and epidemiology. From an epidemiologic point of view, hypothesis is evaluated in one of two ways: either by comparing the hypotheses with the established facts or by using analytic epidemiology to quantify relationships and assess the role of chance.

Step8. Communicate findings

The final task is to summarize the investigation, its findings, and its outcome in a report, and to communicate this report in an effective manner. An oral briefing for local authorities and a written report that follows the usual scientific format of introduction, background, methods, results, discussion, and recommendations should be send to concerned authority.

Step 9: Implement control and prevention measures

In most outbreak investigations, the primary goal is control of the outbreak and prevention of additional cases. In practice control and prevention activities are implemented as early as possible.

Step10: Follow up recommendations/ maintain surveillance

Once control and prevention measures have been implemented, outbreak situation should be monitored. Active surveillance should be continued as it is essential for:

  • Deciding if outbreak is over and for this surveillance may be continued for at least two incubation periods of concerned disease following onset in the last case
  • Documenting effectiveness of control measures

Disease surveillance, outbreak investigation and response in India

Integrated Disease Surveillance Programme, National Centre for Disease Control working under Directorate General of Health Services, Ministry of Health and Family Welfare (MoHFW), Government of India (GOI) is implementing and monitoring the disease surveillance in the country by establishing a decentralized state based surveillance system  (laboratory based and IT enabled)  for epidemic prone diseases to detect the early warning signals, so that timely and effective public health actions can be initiated in response to health challenges in the country at the districts, state and national level.

The Integrated Health Information Platform (IHIP), a web-enabled near-real-time electronic information system is also incorporated for disease surveillance and for managing disease outbreaks since November 2018 in selected districts of 7 States (Karnataka, Andhra Pradesh, Himachal Pradesh, Odisha, Uttar Pradesh, Telangana & Kerala) in its first phase. 

Division of Emergency Medical Relief, MoHFW, GOI is also supporting states/ districts in outbreak investigation and response that requires action at national level and other public health emergencies of international/ national concern.
















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