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How to solve a (virtual) outbreak — everything you need to know

Jessica Hess COVID-19 Review Science

Have you ever wondered how disease outbreaks are investigated?

Who are the brave people in the field collecting and analyzing infectious disease data? And how do health authorities find the source of an outbreak?

In this post I'll break down the actual epidemiological process followed by CDC Disease Detectives and show you how you can become a virtual detective, too!


Outbreaks, Epidemics, and Pandemics

The term epidemic refers to a widespread occurrence of an infectious disease within a community at a particular time. Pandemic, on the other hand, refers to a disease prevalent within a whole country or the world.

Outbreaks occur when there are more cases of a disease than expected. Generally, such cases are assumed to have a shared cause or be related in some way. Outbreaks occur frequently and are typically uncovered through the routine analysis of infectious disease cases sent by labs/healthcare providers to health departments. For instance, the health department team in Oregon revealed an outbreak in 1997 of E. coli strain O157:H7 by spotting infection in three patients who all drank raw milk. As additional surveillance, hospital infection control practitioners and the CDC also regularly review laboratory results of individuals to detect unusual clusters of illness caused by the same organism. For instance, by analyzing statistics from 4 different symptom surveillance systems, the health department in New York discovered a consistent rise in gastroenteritis after an extended blackout in August 2003. Their investigation revealed that the gastroenteritis was probably caused by people consuming meat that had spoiled in the power outage.


How do they decide which outbreaks to investigate further?

Local health departments are more likely to investigate outbreaks when a lot of people have been affected, the disease has a high risk of hospitalization, and when the outbreak has the potential to spread unless prompt control measures are taken. Occurrence of a new or rare disease is also more likely to prompt an investigation than a common disease.

Field investigations cost a lot of money so the decision to investigate must be made judiciously. Investigations are usually justified if they are for one of the following purposes:

  • Control or prevention of the disease
  • To advance research
  • Public, political, or legal concern
  • To strengthen public heath program efforts
  • Training


Once an investigation has been approved, epidemiologists must work quickly and carefully to save lives and prevent the disease from spreading! They follow a standardized approach to ensure that the investigation proceeds without any important steps missed along the way.


Who investigates potential outbreaks?

Disease detectives, also known as Epidemic Intelligence Service (EIS) officers, are highly-trained professionals on the front lines of public health when disease outbreaks emerge. They are responsible for conducting a detailed investigation of health threats, identifying their cause, rapidly implementing control measures, and recommending preventative actions. Disease detectives are doctoral-level scientists, physicians, veterinarians, nurses, or allied health professionals that undergo special training through a 2-year fellowship with the CDC.

Interested in learning more about the role and how to apply? Click here for the EIS main page!


How are outbreak investigations conducted?

The following steps may be performed in a different order than shown, or even several at a time, but all are important to a “quick and clean” outbreak investigation!

  1. The field investigator (disease detective) must prepare for field work — they must have the proper scientific knowledge (read literature), supplies (collection/storage devices), equipment (protective gear), and a plan of action. They must also select a team to accompany them and assist with field responsibilities.
  2. The existence of an outbreak is established — the observed number of disease cases is compared with the expected number from the previous few weeks, months, or a comparable period during a previous year.
  3. The identity of the disease is verified — control measures are often disease-specific, so the disease-causing organism must be confirmed to ensure that laboratory errors were not made. The field investigator does this by reviewing clinical findings, laboratory results, and visiting one or more patients with the disease.
  4. The criteria for diagnosis is defined — this is called a 'case definition'. Diagnoses may be uncertain early in an investigation, so the team categorizes cases as Confirmed, Probable, and Possible/Suspect as indicated by the case definition. The criteria for each diagnostic tier must be applied consistently to all individuals being investigated.
  5. Reports of cases are found and recorded — Investigators may send a letter to local health practitioners to ask for reports of similar cases, survey the population, or visit facilities to collect information on any additional cases. The local media usually becomes aware of the situation at this point.
  6. The outbreak is categorized by time, place, and person — this step may be repeated several times during an investigation as new information becomes available. From this characterization, at risk populations can be identified, hypotheses about etiology, source, and mode of transmission can be formed/tested, and preventative measures can be initiated.
  7. Testable hypotheses are developed — Investigators want to know the disease's usual reservoir, how it's transmitted, what vehicles are commonly implicated, and what the risk factors are. Interviews with patients and healthcare practitioners can provide useful clues that can be turned into hypotheses.
  8. Hypotheses are evaluated for plausibility — typically using a combination of environmental evidence, laboratory tests, and epidemiology. The most common statistical test for outbreak data is the chi-square test.
  9. Hypotheses are reconsidered, refined, and re-evaluated — If no correlation between exposure and disease has been identified, the investigator must reconsider their hypothesis, re-examine the evidence, and conduct additional research. Investigators are interested in acquiring all the knowledge they can about the disease, its modes of transmission, characteristics, and host factors.
  10. Epidemiology, laboratory science, and environmental studies are reconciled — together, these three fields complement each other and can strengthen the conclusions of the outbreak investigation. While in the field, investigators often take photos to document environmental conditions and collect physical evidence for laboratory analysis.
  11. Control and prevention measures are implemented — The health department's first responsibility is to protect public health, so appropriate control measures are initiated as soon as they are known and available. Control measures are usually directed at one or more parts of the chain of transmission (disease agent, source of disease, mode of transmission, portal of entry, or the host).
  12. Surveillance is initiated or maintained — control and prevention measures must be continuously monitored after they are implemented to determine whether they are working or if the outbreak has spread outside its original area.
  13. Findings are communicated — the investigation is summarized and communicated in a written report and/or through an oral briefing for local authorities. The written report is in a format similar to a lab report and contains an introduction, background, methods, results, discussion, and recommendations.


Become a virtual disease detective!

Test your detective skills with the CDC's Solve the Outbreak game.

Your virtual mission, should you choose to accept it, is to analyze clues and data to solve realistic outbreak scenarios from around the world, slow the spread of disease, and save lives! You must act quickly & carefully to successfully climb the ranks from Trainee to Disease Detective.



In summary, the CDC follows a standardized process to investigate disease outbreaks like COVID-19. Disease detectives are on the front lines of the investigation and must be both quick and detailed in their research to stop the disease from spreading. The reports generated from these investigations are used to inform the public and government officials of: an outbreak's severity, who the at-risk populations are, and how the disease came about. Control and prevention measures are initiated as soon as they are known, and are continually revised as disease detectives learn more about the virus or organism they're dealing with. These brave professionals are truly the unsung heroes of public health!




  • CDC, Division of Scientific Education and Professional Development. (2012). Principles of Epidemiology in Public Health Practice (3rd Edition). U.S. Dept of Health and Human Services. https://www.cdc.gov/csels/dsepd/ss1978/SS1978.pdf
  • CDC, Epidemic Intelligence Service. (2020). What EIS Officers Do. U.S. Dept of Health and Human Services. https://www.cdc.gov/eis/what-eis-officers-do/index.html
  • Featured image: Digitally colorized TEM image depicting avian infectious bronchitis virus (IBV) virions, which are Coronaviridae family members. CDC (1975)



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  • Martin Burciaga on

    Wow that is a lot of work from these disease detectives. Its good that they choose people who have more experience in this kind of field to go out and investigate outbreaks. I always knew that there had to be some sort of investigation done in a situation like this but i would’ve never imagined how much work these detectives actually had to do. Its also interesting how veterinarians can qualify to be disease detectives. I also would’ve thought that by the time people are done with uni they wouldn’t need to worry about lab reports, guess i have plenty more of those to look forward to!

  • Sally Quintana on

    Overall my favorite part was learning about the steps it takes for the outbreak investigation to take place. Each and every step that I read I found very interesting due to the fact that a lot of research and commitment takes place as it relates to what is currently happening today with that of Covid-19. I was also very surprised to read that many disease detectives are veterinarians.

  • Lily Johnson on

    My favorite part of this article was when the disease detectives were discussed. I always knew there were people who would go out and investigate during outbreak incidences but aside from that I didn’t know about anything else. Now I know the level of skill and amount of experience it takes to become an EIS officer as well as the amount of training. Knowing that makes me respect them even more than I already do. It was also interesting to find out that you can follow different careers and still be able to work this kind of job if you wanted to.

  • Paola Barrios on

    I was truly amazed at how one of the reasons as to why investigations are done are for political reasons. It truly gets you to think about what gets put in the light depending on something that in my opinion should not have a say. It gets me to think specially about the virus going on right now, with the elections coming up this November.
    Another really interesting point was who can disease detectives can be. I was not aware people like veterinarians and nurses could fall into that type of work. I find bacteria and viruses very interesting. Their growth, spreading and how “smart” they truly can be. As someone who is interested in veterinary medicine, knowing that studying and learning more about outbreaks can also be an option in that type of work is very appealing.

  • Miguel Gutierrez on

    I thought it was interesting how a political concern is a justifiable reason to conduct a field investigation, and I wonder exactly what those concerns are. I would have never imagined that politics play a role in disease investigation. I also found all the actual steps the disease detectives have to take very interesting, and all the safety measures they have to take actually make sense considering they don’t really know what they’re working with or against a lot of the time.

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