The Epidemiological Process

You have already learned about the Scientific Method and its importance in experimentation in other science classes. The Scientific Method of investigating problems allows scientists to use logical steps to arrive at valid conclusions. If all the steps are not followed in the investigation, results and conclusions could be flawed. (Of course, an important step in the scientific method relies on repetition. Results must be reproducible in order to be accepted by the scientific community.)

Epidemiologists are scientists and they use a systematic approach to disease outbreak investigations. Unlike some other scientific investigations, epidemiologists working a disease outbreak know that a speedy solution of the outbreak means fewer lives lost or fewer people who get sick. However, they can’t sacrifice efficiency for speed; getting the wrong answer at any step in the investigation would mean the outbreak continues and probably spreads to a larger population.

As you read about the steps in the epidemiological process of investigating disease, think about the reason for each step. Each step is essential. They may just seem like logical steps to you (and they are), but disease was not always investigated in such logical ways. We needed a much better understanding of infectious disease pathogens and disease transmission methods before scientists could define a better way of solving outbreaks. A lack of knowledge still impacts outbreak investigations today; during the Ebola epidemic of 2014-15, many native people did not practice safety procedures with infected patients because they believed that the illness was caused by a curse, not by a virus. Thousands of deaths could have been avoided.



In order to understand the steps in an outbreak investigation, you should be able to:

  • Explain the steps for conducting an epidemiological investigation of an outbreak.
  • Describe the importance of each step within the process and the consequences of not implementing each step.



analytic epidemiology – the field of epidemiology that studies why and how health-related events could occur, including the rates of disease occurrence differences in demographics, behaviors, environmental exposures, and other risk factors.

case definition – in epidemiology, a description of specific conditions and details that should be met in order for the person to be identified as having a particular disease or other health condition.

descriptive epidemiology – the field of epidemiology that organizes and summarizes data regarding persons who are affected by a disease or conditions, time of onset, and location.

epi-curve – a graph that plots the course of an outbreak by date of onset of symptoms.

epidemic – numbers of disease cases, injuries, or other health conditions greater than expected in an area or population during a particular period.

epidemiology – the branch of medicine that studies patterns and causes of health-related conditions and uses those studies and results to control a health problem.

histogram – a graph using bars or columns to represent values in a statistical study; in epidemiology, time is usually plotted on the x-axis with number of cases on the y-axis.

line listing – in epidemiology, a spreadsheet list of cases or patients with information about each patient, including gender, age, and date of onset of symptoms.

outbreak – the sudden rise in the occurrence of a disease or injury in a given area during a given time period; generally considered to be more localized than an epidemic.


Steps for Conducting An Epidemiological Investigation

The Centers of Disease Control and Prevention (CDC) has identified the epidemiologic steps of an Outbreak Investigation. In an actual outbreak investigation, some of the steps may occur simultaneously as epidemiologists work on solving and controlling the spread of disease.


1. Prepare for field work and establish the existence of an outbreak.

An outbreak is the sudden rise in the occurrence of a disease in a given area during a given time period. An outbreak is generally considered to be more localized than an epidemic. An epidemic is the occurrence of more cases of a disease than would normally be expected in an area or population during a particular period; epidemics involve more cases than outbreaks and occur over a wider area.

The first task is to verify if an outbreak is occurring. Usually, case data that comes into a national healthcare organization like the National Institutes of Health (NIH) or the Centers for Disease Control (CDC) have already been analyzed for frequency of occurrence. Unusual cases that do not exceed the normally expected values may be investigated as well.

Preparation for field work involves gathering the necessary supplies, equipment, transportation arrangements, information about similar outbreaks, journal articles, sample questionnaires, and personnel that may be particularly qualified to handle this disease. Field work is not always necessary if investigators can perform the necessary analysis from their labs and offices and there are appropriate health officials on the scene.

Note: If cases are increasing, but are not more than would normally be expected by health department data or national health records, it would not be classed as an outbreak.


2. Verify the diagnosis.

Treatment plans, control measures, and prevention strategies are disease-specific. If the disease is not correctly identified, treatment plans will not help patients recover and control plans will not contain outbreak.

Epidemiologists order clinical tests in the field or review results done by field hospitals and laboratories to make sure the diagnosis is correct. They also conduct patient interviews to gain a better understanding of the disease process, the incubation period, and how the patient may have been infected. They ask about patient activities in order to identify people who may also be infected or if the patient has spread the disease to other people.


In the 2014-15 Ebola outbreak the disease was incorrectly identified as cholera and malaria for three months. Many more people died and the outbreak spread to several countries before it could be contained.


3. Construct a working case definition.

A case definition is a description of a set of criteria that is used to include or eliminate patient’s symptoms as having the health condition in the outbreak. The description is based on objective criteria, such as degree of fever, presence of rash, description of the type of rash, or severity and length of headache.

The case definition also includes the time of onset of disease symptoms and location of the patient.

Epidemiologists often classify a case definition as confirmed, probable, or possible in the early stages of the investigation. Once positive laboratory test results are obtained, the case definition is classified as confirmed. The case definition is then used to determine if other people afflicted with the same symptoms are included in outbreak or epidemic totals.


4. Find cases and record data.

The cases that are recorded by health departments are often a small part of the actual total number of cases. In order to get a clear picture of the outbreak, epidemiologists must look for possible additional cases by asking for reports from public health agencies and healthcare facilities such as clinics and hospitals. This is called passive surveillance. They also go directly to the public to solicit information about the illness in the community (called active surveillance). Patients are a good source of information concerning other contacts, if the condition is infectious.

The data is organized into the case report forms which detail symptoms and patient demographics, such as gender and age. The case reports become part of a larger line listing which includes similar information but many more cases.

This table is part of a line listing of an anthrax investigation.


Source: Jernigan DB, Raghunathan PL, Bell BP, Brechner R, Bresnitz EA, Butler JC, et al. Investigation of bioterrorism-related anthrax, United States, 2001: epidemiologic findings. Emerg Infect Dis 2002;8:1019–28.

Example of a case form for a patient with Lyme Disease.









5. Analyze the data as to time, person, and place.

Called descriptive epidemiology, this step yields valuable clues about the disease. Identifying the pathogen does not always mean that you know how it was transmitted; the same pathogen can be transmitted in a number of different ways. An analysis of time can provide clues about incubation periods and trends can determine the infectivity of a viral strain.

Compiling demographic information can provide insights, such as if some populations are more affected than others. Analysis of data also includes tables and histograms for age and gender and an epi-curve that provides a graphical display of the outbreak over time. An epi-curve graphs the number of cases and time of onset.


Mapping cases can provide valuable clues to transmission, sources of contamination, local water supplies, or other characteristics which might allow epidemiologists to control the outbreak.

(For more information or to practice constructing epi curves, see Creating and Interpreting an Epi-curve.)


6. Develop hypotheses.

Although developing a hypothesis is usually listed as a step after gathering and graphing the case data, epidemiologists usually work a few of these steps simultaneously. While they are reviewing case reports and line listings, for example, they are often thinking about what could be the cause of the outbreak (the hypothesis). As each piece of the outbreak puzzle presents itself to investigators, they are constantly revising and reorganizing what they know in order to resolve the situation.

Epidemiologists often talk to case patients, local healthcare providers, local health department personnel, and patient contacts in order to gather information that will help develop their hypothesis.


7. Test the hypothesis epidemiologically. Refine and re-evaluate as necessary.

Even if the hypothesis is correct, a more specific hypothesis is sometimes needed in order to yield more information on transmission or source of the outbreak. Epidemiologists use many more case reports, interview as many patients as possible and consider multiple variables to test specific outbreak factors.

Analytical epidemiology studies the rates of disease occurrence, differences in demographics, behaviors, environmental exposures, and other risk factors involved in the outbreak disease. It adds to or supports descriptive epidemiology in resolving an investigation.


8. Check your hypothesis with laboratory results and environmental study data.

At this stage of the investigation, laboratory results are usually available and other studies that have been on-going are yielding data. It sometimes takes weeks to obtain lab results of tests that require growing bacterial or viral cultures and multiple steps to perform in a lab hundreds, if not thousands of miles away. During this time, the investigation has continued and data from all steps can be analyzed. The hypothesis as to the cause can be confirmed.


9. Plan and implement control measures.

This is another step that is often done at the same time as other steps in an epidemiological investigation. If the cause of the outbreak is reasonably thought to be water-borne, it is a good idea to stop people from drinking the water even though definitive test results have not been received; if the outbreak is thought to be spread person-to-person through respiratory secretions, then a public warning about spread of the pathogen is a good control measure. Sometimes, control measures are started even before an investigation if the cause and effective controls are not in doubt. The first priority of health departments is to protect public health.

Prevention and control measures should be evaluated continually. If the outbreak cases continue to increase, prevention and control measures are ineffective and should be revised.


10. Maintain surveillance.

Surveillance is continued to ensure that prevention and control measures are effective and to make sure that the outbreak has not spread to more areas. If the outbreak spreads, control measures must be started in those additional areas.


11. Communicate findings.

Outbreak control also depends on public awareness. Public health alerts inform people of the infectious agent, the scope of the outbreak, and prevention and control measures being taken to control the outbreak. Public health information should be updated and thorough records kept of each outbreak. Information should be disseminated to state and local public health departments and poison control centers.

Check Your Understanding:

  1. What is the importance of following the epidemiological method when investigating possible disease outbreaks?
  2. A sudden rash of similar symptoms is reported to a county health department by several hospital emergency rooms, doctors’ offices, and healthcare facilities.
    1. Write a case definition for the following illness:
      • Representative symptoms of 28 cases of disease received by a county health department in Ohio:
        • Severe diarrhea, vomiting, abdominal pain for last 3 days
        • Bloody diarrhea, vomiting, abdominal cramping, low-grade fever for 4 days
        • Diarrhea, vomiting, abdominal pain, dark urine, low urine output for last 2 days
        • Severe diarrhea, blood in stools, abdominal pain, bloody urine, low-grade fever for last 3 days; symptoms getting worse
        • Diarrhea, vomiting, abdominal cramping for 5 days; symptoms are improving
      • Five people have been hospitalized due to dehydration.
        • Preliminary case definition:
        • Critical thinking: What are possible hypotheses for these symptoms?
        • How would you test these hypotheses?
  3. You find out the illnesses were food poisoning by E. coli and that the source was cheese bought at a large grocery store. The cheese was supplied by a major distributor which supplies cheese to multiple states in the Mid-west region.
    1. Write a public health report which will alert the public to the E. coli outbreak. Your public health report should include details about symptoms of the disease, cause, how to avoid the disease, and what to do if you get sick.