Legionnaires' pneumonia is not unusual. In fact, it represents over 4% of all community-acquired pneumonias. An additional unknown number of people are infected with the Legionella bacterium but have only mild symptoms or no symptoms at all (so-called Pontiac fever).Outbreaks of Legionnaires' disease have received the most media attention. A disease outbreak was associated with a party at the Playboy mansion in Los Angeles where at least four individuals contracted Legionnaires' disease. However, the disease most often occurs as single, isolated cases not associated with any identified outbreak. Outbreaks are usually recognized in the summer and early fall, but cases may occur all year long. More recently, outbreaks have been detected in gym participants in Florida and in a police station in New York.Since the bacterium of Legionnaires' disease was identified in 1976, numerous hospital-acquired outbreaks of the disease have also been reported. These outbreaks have enabled health officials to study epidemics of legionellosis. A recent CDC evaluation reported that one in five Legionnaires' disease cases are linked to health care facilities.