Acquired by inhalation of aerosolized bacteria or, rarely, microaspiration of contaminated drinking water.
Presentation includes respiratory symptoms such as cough (may not be productive) and shortness of breath, fever, chills, and chest pain. Other symptoms include headache, nausea, vomiting, abdominal pain, or diarrhea.
Definitive diagnosis is made with a positive laboratory test.
First-line treatment is with a fluoroquinolone or a newer-generation macrolide.
Complications are the same as those for pneumonia and include respiratory failure, parapneumonic effusion, empyema, lung abscess, and death.
Legionella are gram-negative rod bacteria with at least 20 pathogenic species and 70 serogroups. Legionella reproduce intracellularly, including within macrophages, which paradoxically protect them as they replicate.  Legionella pneumonia, known as Legionnaires' disease, occurs when the bacteria are inhaled (or rarely aspirated) into the lungs. Nonpneumonic Legionella infection is known as Pontiac fever.
Associate Professor of Medicine
Division of Infectious Diseases
Department of Medicine
School of Medicine
University of Louisville
FWA declares that he has no competing interests.
Dr Forest W. Arnold would like to gratefully acknowledge Dr Allen Griffin, a previous contributor to this topic. AG declares that he has no competing interests.
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