Legionellosis refers to Legionnaires' disease, also known as "Legion Fever," which is the more severe form of the infection and produces pneumonia and Pontiac fever, which is caused by the same bacteria but produces a milder respiratory illness without pneumonia that resembles acute influenza.
Legionnaires' disease acquired its name in July 1976 when an outbreak of pneumonia occurred among people attending a convention of the American Legion at the Bellevue-Stratford Hotel in Philadelphia. The next year, the causative agent was identified as a previously unknown strain of bacteria, subsequently named Legionella.
"Older individuals and people living in the Northeast are most at risk for developing legionellosis, although legionellosis occurred in all age groups and regions," the Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report says.
CDC officials are working with state health departments to determine why the number of reported legionellosis cases is increasing, the report says.