Legionnaires' disease, or legionellosis, is caused by the bacterium Legionnella. The freshwater bacterium is usually low risk for humans, but chance of infection becomes greater when Legionella is able to multiply in indoor water systems. The disease is not contagious from person-to-person exposure; it is usually caught from inhaling microscopic water droplets in mist or vapor. Infection causes severe pneumonia in some, usually people over 50-years-old or possessing a weakened immune system. Those affected in Anaheim ranged from the ages of 52 to 94, according to Jessica Good, a spokeswoman for the Orange County Health Care Agency.