As specialized laboratory tests became more widely available, Legionella species were found to be common causes of nosocomial and community-acquired pneumonia. Patients with chronic lung disease and organ transplants are at greatest risk. Clinical manifestations are non-specific, although fever greater than 39 degrees C and diarrhea are common. Erythromycin remains the antibiotic of choice, although many alternative agents are available. Once cases are discovered, a search for the organism in water distribution systems and respiratory equipment can be fruitful. Disinfection of water distribution systems by superheating and flushing or by hyperchlorination is feasible.