Legionellosis is a respiratory infection caused by Legionella bacteria, most commonly Legionella pneumophila. These bacteria are transmitted by contaminated water aerosols, which are then inhaled. Those at greatest risk of acquiring infection are those over age 50, especially with a history of smoking, and those with weakened immune systems. The severe form of legionellosis, commonly known as "Legionnaires’ Disease", is characterized by pneumonia, fever, and myalgia. A milder, self-limiting form of the illness, known as Pontiac Fever, is characterized by fever, cough, and myalgia. Neither infection is transmissible person-to-person.

There were 23 cases of legionellosis reported in Indiana in 2001, for a rate of 0.4 cases per 100,000 population. This represents a 44% decrease over the previous year. In addition, the incidence was well below the previous five-year average incidence of 47 cases (Figure Leg1). Legionellosis infection occurs any time of year, and there does not appear to be any seasonal pattern.

Age-specific rates were highest among adults ages 60-69 (1.4) and adults ages 70 and older (1.1) (Figure Leg2). The sex-specific rate for males (0.5) was higher than that for females (0.3).  The race-specific rate for blacks (1.0) was higher than that for whites (0.3).

There were no outbreaks of legionellosis reported in Indiana in 2001.