Steps in a Foodborne Outbreak Investigation

A foodborne outbreak investigation goes through several steps. They are described here in order, but in reality investigations are dynamic and several steps may happen at the same time.

Click on each step to learn more about the investigation process.

Illustration representing detecting a sickness by showing a stethoscope and a thermometer.

Step 1: Detect a Possible Outbreak

Detecting an outbreak is the first step in investigating a multistate foodborne outbreak.

  • An outbreak with multiple sick people can be missed if they are spread out over a wide area.
  • Outbreaks are detected by using public health surveillance methods, including PulseNet, formal reports of illnesses, and informal reports of illnesses.

Illustration showing the United States and a magnifying glass representing finding more cases.

Step 2: Define and Find Cases

Finding sick people is important to help public health officials understand the size, timing, severity, and possible sources of an outbreak.

  • A case definition is developed to define who will be included as part of an outbreak.
  • Investigators use the case definition to search for illnesses related to the outbreak.
  • Illnesses are plotted on an epidemic curve (epi curve) so that public health officials can track when illnesses occur over time.

Illustration representing interviewing people that are sick by showing a telephone

Step 3: Generate Hypotheses about Likely Sources

Hypothesis generation is an ongoing process.

  • Possible explanations of an outbreak are continually changed or disproved as more information is gathered.
  • Interviews, questionnaires, and home visits are helpful in narrowing down how and where people in the outbreak got sick.

Illustration representing testing by showing a microscope.

Step 4: Test Hypotheses

A hypothesis is tested to determine if the outbreak source has been correctly identified.

  • Investigators use many methods to test their hypotheses.
  • Two main methods are analytic epidemiologic studies and food testing.

Illustration representing the source and movement of outbreak by showing a farm, home, shopping cart and a restaurant.

Step 5: Solve Point of Contamination and Source of the Food

Health officials use three types of data to link illnesses to contaminated foods and solve outbreaks: epidemiologic, traceback, and food and environmental testing.

  • Health officials assess all of these types of data together to try to find the likely source of the outbreak.
  • A contamination can happen anywhere along the chain of food production, processing, transportation, handling, and preparation.

Illustration representing ways to control an outbreak. This one has a spray bottle spraying disinfectant.

Step 6: Control an Outbreak

Once the food source of an outbreak is determined, control measures must be taken.

  • If contaminated food stays on store shelves, in restaurant kitchens, or in home pantries, more people may get sick.
  • There are several different outbreak control measures that can be taken.
  • Public health officials choose measures based on the information available to them.
  • Measures can change as the investigation goes on.

illustration representing the decline of cases in an outbreak by using a column chart.

Step 7: Decide an Outbreak is Over

An outbreak is over when the number of new illnesses drops back to what investigators normally expect.

  • With continued public health surveillance, if the number of illnesses rises again, the investigation continues or restarts.