About the Data: Case Definitions

Key points

  • Use the most recent case definition when reporting cases of legionellosis, which includes Legionnaires' disease and Pontiac fever.
  • Reporting potential exposures can be important for identifying investigating outbreaks.

Case definition

The Council of State and Territorial Epidemiologists (CSTE) released the most recent case definition for national legionellosis surveillance in 2020.

It includes information on the following:

  • Clinical criteria
  • Laboratory criteria
  • Epidemiologic linkage
  • Criteria to distinguish a new case from an existing case
  • Case classification

Case classifications

Traditionally, the probable case definition was used in outbreak investigations to aid in the identification of potential sources of exposure. The updated CSTE position statement includes new language to help public health investigators better categorize these types of legionellosis cases.

Confirmed and suspect are CSTE surveillance case classifications, based on type of laboratory testing and clinical presentation.

Probable is a CSTE surveillance case classifiction, based on epidemiology linkage and clinical presentation in absence of laboratory testing.

Presumptive and possible are healthcare exposure classifications and can apply to confirmed, suspect, and probable cases.

Exposure categories

Sometimes the date of symptom onset is difficult to define (e.g., in patients with chronic underlying illness). Date determination is deferred to the judgment of

  • Public health officials performing the investigation
  • Clinicians providing care


Travel-associated case: Patient spent at least one night away from home in the 14 days before symptom onset. This includes nights in the state of residence, another state, or another country. It doesn't include nights spent in a healthcare facility.



Presumptive healthcare-associated case: ≥10 days of continuous stay at a healthcare facility during the 14 days before symptom onset. The case is considered a presumptive healthcare-associated case for surveillance purposes even if the stay involved multiple healthcare facilities.


Possible healthcare-associated case: A portion of the 14 days before onset of symptoms spent in one or more healthcare facilities. These cases don't meet the criteria for presumptive healthcare-associated Legionnaires' disease.

High-risk settings

Although there aren't specific case classifications for other exposure types, assisted living, senior living, and correctional facilities often house at-risk populations. They can also have large, complex building water systems. Consider these as likely sources as residents may have limited or no exposures outside the facilities. These facilities should all have water management programs.

Multiple exposures

Patients can have both travel and healthcare exposures in the 14 days before onset of symptoms. The case report forms provide space to indicate multiple exposures.

Data terminology

Legionellosis vs Legionnaires' disease

For surveillance purposes, CDC uses the term "legionellosis" to ensure all forms of clinical disease due to Legionella are included:

  • Legionnaires' disease
  • Pontiac fever
  • Extrapulmonary disease

Elsewhere on this website, though, CDC refers to "Legionnaires' disease" instead of "legionellosis" when describing cases and outbreaks. One reason for this is the majority (98%) of cases reported to the Supplemental Legionnaires' Disease Surveillance System are Legionnaires' disease. In addition, prevention efforts are often designed with Legionnaires' disease in mind due to its association with substantial mortality.

Pontiac fever

Nevertheless, Pontiac fever deserves attention. Outbreaks of Pontiac fever can be large and can place burden on the medical system. Furthermore, Pontiac fever can signal the presence of conditions that support Legionella growth and transmission. Environmental sources that lead to cases of Pontiac fever are often also associated with cases of Legionnaires' disease.

Extrapulmonary infections

Although extremely rare, Legionella can also cause extrapulmonary infections, such as endocarditis or wound infections.