Legionnaires’ Disease, severe form of pneumonia, characterized by headache, chest pain, lung congestion, and high fever. The name is derived from an outbreak at an American Legion convention in a Philadelphia hotel in July 1976. The United States Centers for Disease Control and Prevention (CDC) later succeeded in identifying the causative agent as a previously unknown rod-shaped bacterium (Legionella pneumophila). Other local outbreaks were then linked to the disease. An estimated 8,000 to 18,000 people contract Legionnaires' disease in the United States each year, and about 5 to 30 percent of affected people die. The bacterium is most commonly transmitted through contaminated water sources; it is not readily transmitted from person to person. Treatment with the antibiotic erythromycin is effective.