ATLANTA — According to officials from the Centers for Disease Control and Prevention (CDC), the number of legionellosis cases reported each year to the CDC among United States residents more than tripled between 2000 and 2009 from 1,110 cases to 3,522 cases.

Older individuals and people living in the Northeast are most at risk for developing legionellosis, although legionellosis occurred in all age groups and regions. Legionellosis can refer to either Legionnaires disease, a serious, sometimes lethal pneumonia, or Pontiac fever, a flu-like, self-limited illness, but almost all of the cases reported to CDC were Legionnaires disease.

CDC said it is working with state health departments to determine why the number of reported legionellosis cases is increasing. Rising numbers of elderly persons and other persons at high-risk for infection as well as increased case detection or reporting may have contributed to the number of cases.