Legionellosis is associated with three clinically and epidemiologically distinct illnesses: Legionnaires’ disease, Pontiac fever, or extrapulmonary legionellosis.

Legionnaires’ disease: Legionnaires’ disease presents as pneumonia, diagnosed clinically and/or radiographically. Evidence of clinically compatible disease can be determined several ways: a) a clinical or radiographic diagnosis of pneumonia in the medical record OR b) if “pneumonia” is not recorded explicitly, a description of clinical symptoms that are consistent with a diagnosis of pneumonia.

Pontiac fever: Pontiac fever is a milder illness. While symptoms of Pontiac fever could appear similar to those described for Legionnaires’ disease, there are distinguishing clinical features. Pontiac fever does not present as pneumonia. It is less severe than Legionnaires’ disease, rarely requiring hospitalization. Pontiac fever is self-limited, meaning it resolves without antibiotic treatment.

Extrapulmonary legionellosis: Legionella can cause disease at sites outside the lungs (for example, associated with endocarditis, wound infection, joint infection, graft infection). A diagnosis of extrapulmonary legionellosis is made when there is clinical evidence of disease at an extrapulmonary site and diagnostic testing indicates evidence of Legionella at that site.