Earn rewards with every purchase! Click the blue ⚡ to join
Earn rewards with every purchase! Click the blue ⚡ to join
Cart 0

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)



Newer Post

  • Samantha Vargas on

    I really enjoyed reading this article. I think I could take it to apply to my daily life now with everything that is going on with the COVID-19 situation. The GIFs included in the article made it really fun to read. I give all my respect to the scientists and investigators who are working non-stop on the research of these new an existent viruses and diseases.

  • Evelin carrasco on

    I absolutely loved learning about the Disease detectives, also known as Epidemic Intelligence Service (EIS) officers. It was great to learn about what they do and who they are as well as learning what steps they have to take in order investigate a disease. I never realized that this career was out there until reading and living through this pandemic.

  • Karen S Chavez on

    I really enjoyed reading this article especially with COVID-19 going on, there’s a lot of misinformation about the CDC. I did not know that there were so many steps on how an outbreak investigation is conducted. Overall, it was very informative and the content was clear. The gifs made it more interesting. I enjoyed this article.

  • Miriam R. Escobedo on

    Considering what the world is currently going through, this article was extremely informative and explained a lot of questions that I have had recently about how disease outbreaks are investigated. My favorite part of this blog is the section regarding the breakdown process which explains how outbreak investigations are conducted. There was a lot more of a vigorous and meticulous process than I had imagined. I learned that there is a lot of tedious research that goes into conducting investigations for outbreaks, especially the first step which requires a lot of fieldwork and planning before the actual identification of the outbreak can be established. While I greatly enjoyed reading this blog, I also had a lot of fun playing the CDC’s “Solve the Outbreak game” which provided me with greater insight as to how disease detectives work and visualizing the process of a disease outbreak investigation.

  • Estephany Rodriguez on

    The information is laid out amazingly!
    it’s not difficult to follow along and understand, I especially like how they include humorous GIFs. What also stuck out to me is how much the CDC truly does and how they don’t get as much recognition as they deserve, many blame them for a lot thats out of their control, and don’t realize all the work they put in every single step in their research, their work is outstanding.

Leave a comment

Please note, comments must be approved before they are published