Legionnaires' disease.

The Legionella bacillus is a relatively common pulmonary pathogen that has been responsible for a number of outbreaks of respiratory illness this century. Not until 1976, however, after exciting epidemiologic and microbiologic investigation, was the organism isolated and identified. Legionnaires' disease does not have a characteristic radiographic appearance, but certain features may alert the clinician to its presence. It often rapidly progresses to a lobar pneumonia that may not respond immediately to treatment. The radiographic findings lag behind clinical improvement, and radiographic resolution is prolonged. Organ transplantation patients often present with ill-defined, rounded, pleura-based opacities that may simulate pulmonary infarction and can cavitate.