Clinical Features

Legionellosis is a bacterial disease caused by Legionella that can present as either Legionnaires’ disease or Pontiac fever. Legionnaires’ disease causes severe pneumonia requiring hospitalization, while Pontiac fever generally resolves on its own. Although extremely rare, Legionella can also cause extrapulmonary infections, such as endocarditis or wound infections.

The following table summarizes key clinical differences between Legionnaires’ disease and Pontiac fever.

Table summarizes key clinical differences between Legionnaires’ disease and Pontiac fever
Legionnaires’ disease Pontiac fever
Clinical features Fever, myalgia, and cough (according to the CSTE* case definition)

These symptoms are typical but not required; additional symptoms (e.g., shortness of breath, headache, confusion, nausea, diarrhea) may be present

A milder illness without pneumonia (according to the CSTE* case definition)

A flu-like illness, often with fever, chills, headache, myalgia, fatigue, malaise; less often with symptoms such as cough or nausea

Pneumonia (clinical or radiographic) Yes No
Pathogenesis Replication of organism Possibly an inflammatory response to endotoxin
Incubation period 2 to 10** days after exposure 24 to 72 hours after exposure
Percent of people who become ill, when exposed to the source of Legionella Less than 5%2 Greater than 90%3
Treatment Antibiotics Supportive care (because illness is self-limited)
Isolation of the organism Possible Never demonstrated
Outcome Hospitalization common
Case-fatality rate: 10% (25% for healthcare-associated)
Hospitalization uncommon
Case fatality rate: extremely low

* CSTE: Council of State and Territorial Epidemiologists
** The incubation period for Legionnaires’ disease is most commonly 2 to 10 days from the time of exposure to symptom onset, with an average of 5 to 6 days1, but public health officials have reported incubation periods up to 26 days under rare circumstances2. For surveillance purposes, public health officials collect exposure histories for the 10 days before date of symptom onset. However, in outbreak settings where it is important to consider a wide range of possible sources, use of a 14-day range (or more) may be desirable.


  1. Egan JR, Hall IM, Lemon DJ, Leach S. Modeling Legionnaires’ disease outbreaks: Estimating the timing of an aerosolized release using symptom-onset datesexternal icon. Epidemiology. 2011;22(2):188–98
  2. Fraser DW, Tsai TR, Orenstein W, et al. Legionnaires’ disease: description of an epidemic of pneumoniaexternal icon. N Engl J Med. 1977;297(22):1189–97.
  3. Glick TH, Gregg MB, Berman B, et al. Pontiac fever. An epidemic of unknown etiology in a health department: I. Clinical and epidemiologic aspectsexternal icon. Am J Epidemiol. 1978;107(2):149–60.

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