Legionellosis manifests as either an acute, febrile, self-limited illness (Pontiac fever) or pneumonia (Legionnaires' disease).

• Pontiac fever is a flulike illness with a 24- to 48-h incubation period. Malaise, fatigue, and myalgias occur in 97% of cases. Fever, chills, and headaches are also very common, but pneumonia does not develop. The disease is self-limited and does not require antimicrobial treatment. Recovery takes place in a few days.
• Legionnaires' disease is more severe than other atypical pneumonias and is more likely to result in ICU admission.
  • - After a usual incubation period of 2-10 days, nonspecific symptoms (e.g., fever, malaise, fatigue, headache, anorexia) develop and are followed by a cough that is usually mild and only slightly productive. Chest pain and GI difficulties can be prominent.
  • - Radiologic findings are nonspecific, but pleural effusions are present in 28-63% of pts on hospital admission.
  • - Legionnaires' disease is not readily distinguishable from pneumonia of other etiologies based on clinical manifestations, but diarrhea, confusion, temperatures >39°C (102.2°F), hyponatremia, increased aminotransferase levels, hematuria, hypophosphatemia, and elevated creatine phosphokinase levels are documented more frequently than in other pneumonias.
  • - Extrapulmonary infection results from hematogenous dissemination and most commonly affects the heart (e.g., myocarditis, pericarditis).