How CDC Investigates New Clusters of Illness

Investigating the occurrence of more than the expected number of cases of an illness is a multistep process focused on answering three questions:

  • “What is the problem?”
  • “What is the cause?”
  • “What can we do about it?”

What is the problem?

To identify the problem, CDC creates a “case definition.” A case definition provides a common understanding of who has the illness and helps investigators as they collect data on those cases. The data collected helps scientists understand the size, timing, severity, and possible cause of the illness. Investigators keep track of the cases over time.

What is the cause?

The next step is to develop and assess theories about how or what might be happening to cause people to get sick around the same time. Developing theories about the cause of the illness is an ongoing process. Investigators use interviews, questionnaires, and other tools to help narrow down how and where people got sick. Theories about the cause of an illness are continually changed or disproved as more information is gathered.

What can we do about it?

As investigators identify the likely cause of the illness, they implement disease control and prevention activities to prevent the spread of the disease. Scientists continue to count the number of new cases and monitor the situation to determine if prevention measures are working and disease spread is being controlled.

Public health officials choose measures to control the spread of the illness based on the information available to them. Measures can change as the investigation goes on.

NCIRD Investigations

Learn about current investigations being led by CDC’s National Center for Immunization and Respiratory Diseases (NCIRD).

Page last reviewed: May 6, 2022