Legionnaires’ disease consists of severe pulmonary infection with the aerobic gram-negative bacillus known as Legionella pneumophila. The organism is found in aquatic environments such as humidifiers, cooling towers, reservoirs, and water distribution systems. Infection with this agent results from exposure to these sources, not from person to person contact as is usually the case with pneumonia-causing organisms. The bacterium and the associated pneumonia was named for the unfortunate outbreak at an American Legion convention in 1976 where 29 of the 182 delegates died.

Infection with L. pneumophila causes malaise, myalgia, high fevers, abdominal and chest pain, rigors, dyspnea, and a productive cough. If the patient becomes bacteremic, brain abscesses, pancreatitis, endocarditis, and sinusitis can develop. Patients with chronic bronchitis or emphysema, as well as those with renal failure or a malignancy are predisposed to contracting this illness. Since the organism is difficult to culture, the diagnosis is established serologically; unfortunately the patients require 2 weeks for seroconversion.