Legionella is a family of gram negative coccobacilli. Incidence of Legionella pneumonia in the U.S. is uncertain with estimates ranging from 13,000 to 250,000 cases per year. The first outbreak of Legionella was recognized in Philadelphia at a Legionnaires convention in 1976. Transmission of the disease is airborne, usually through moist air exhaust. Incubation period is estimated to range from 2 to 10 days. Clinical features include: (1) acute febrile illness without pneumonia (2) systemic disease with primarily pulmonary manifestations (3) a peak incidence in patients over 60 years of age (4) predisposition in smokers, COPD, and alcoholic liver disease (5) high fever, shaking chills, cough productive of small amounts of sputum (6) pleuritic chest pain (7) watery diarrhea in 50% of patients (8) headache. The characteristic radiologic finding is air space consolidation that is initially peripheral and sublobar. In many cases, the area of consolidation enlarges to occupy all of a lobe or to involve contiguous lobes of the ipsilateral side. Progression is rapid with most of the lobe becoming involved within 3-4 days. Laboratory findings include a WBC count of less than 15,000 with a left shift. Confirmation of diagnosis is through indirect immunoflourescent testing or antigen detection by DNA probe. One third of patients require assisted ventilation with a mortality rate reported from 10 to 25%. With erythromycin therapy the case mortality rate is below 5%.