Background
Legionnaires disease (LD) was recognized in 1976 after an outbreak of pneumonia at an American Legion convention in Philadelphia. Soon after, the etiologic agent was identified as a fastidious gram-negative bacillus and named Legionella pneumophila. Although several other species of the genus Legionella were subsequently identified, L pneumophila is the most frequent cause of human legionellosis and a relatively common cause of community-acquired and nosocomial pneumonia in adults. In children, L pneumophila is also an important, although relatively uncommon, cause of pneumonia.
Legionellosis refers to 2 distinct clinical syndromes: Legionnaires disease, which most often manifests as severe pneumonia accompanied by multisystemic disease, and Pontiac fever, which is an acute, febrile, self-limited, viral-like illness.