ATLANTA -- Cases of Legionnaires' disease have tripled in the last decade, U.S. health officials said Thursday, but the risk of dying from it is lower because of more effective treatment.
The illness most often strikes the elderly and can cause deadly pneumonia.
The germ spreads through mist or vapor from contaminated water or air-conditioning systems.
The Centers for Disease Control and Prevention reports 3,522 cases in 2009, the most since Legionnaires was first identified in 1976. In 2000, only 1,110 cases were reported.
CDC officials say the increase may be partly because of more senior citizens.
Legionnaires is still rare: Just 8 percent of its victims died in the last decade, compared to 20 percent in the 1980s and 1990s. But still, hundreds of Americans die each year. Despite the low case count, experts say they think the disease sickens and even hospitalizes thousands every year whose cases aren't reported.
The rise in cases is worrisome, study co-author Dr. Lee Hampton, a CDC epidemiologist, said. "We need to minimize the risk of people dying from this."
The disease got its name from an outbreak at a Philadelphia convention of the American Legion in 1976, when more than 200 people fell ill and 34 died.
The outbreak drew intense media coverage, and months later health investigators identified the bacterial cause. The germ apparently had spread through the convention hotel's air-conditioning system.
Early signs of the disease can include high fever, chills, and a cough. Fortunately, some of the drugs most commonly used against pneumonia are first-line treatments against Legionnaires.
Cases of the disease held relatively steady in the 1980s-1990s, but rose since 2000.
The CDC relies on doctors, hospitals, and state health departments to report cases, and agency officials say they believe the national case count is an underestimate.