Step 2: Define and Find Cases

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

Key words:

Case Definition: A list of criteria used to determine if an individual is included as a case in an outbreak investigation

Case Count: The number of illnesses that meet the case definition

Epidemic Curve (Epi Curve): A graph that shows the number of illnesses over time

What is a case definition?

Case definitions are developed by health officials to spell out who will be included as part of the outbreak. Case definitions may include details about:

  • Features of the illness
  • DNA fingerprint (if the pathogen is tracked by PulseNet)
  • The pathogen or toxin (if known)
  • Certain symptoms typical for that pathogen or toxin
  • Time range for when the illnesses occurred
  • Geographic range, such as residency in a state or region

There might be several case definitions for an outbreak investigation, each with a different purpose. For example, one case definition might be for confirmed illnesses and another for probable illnesses. The number of illnesses that meet the case definition is called the case count.

How are cases of illness found?

Using the case definition, investigators search for more illnesses related to the outbreak. They do this by:

  • Reviewing laboratory reports to PulseNet
  • Reviewing regular surveillance reports
  • Asking local clinical and laboratory professionals to report cases of the particular illness more quickly, as soon as they suspect the diagnosis
  • Reviewing emergency room records for similar illnesses
  • Surveying groups that may have been exposed
  • Asking health officials in surrounding areas to look for illnesses that might be related

How are cases tracked?

Investigators watch the progression of an outbreak by tracking:

  • Who becomes ill
  • When they become ill
  • Where they live

Investigators use a graph called an epidemic curve or epi curve to track the number of illnesses over time. The pattern of the epi curve helps investigators decide if sick people were exposed to the same source of illness over a short period or if the exposure to the source was over a longer time. Investigators use maps to mark where sick people live so they can easily see whether and how the outbreak is spreading.

This is a sample epi curve. IT contains the number of people and dates when they became ill. It also lists dates when 2 recalls were made. Dates on the curve range from July 1, 2015 through March 13, 2016.

Example of an epi curve tracking cases and events in an outbreak.