Legionella bacteria can cause a serious type of pneumonia (lung infection) called Legionnaires' disease as well as a milder illness called Pontiac fever. The term "legionellosis" refers to all illnesses caused by Legionella bacteria. People become infected with Legionella when they inhale the bacteria in water mists or vapors (such as those from showers, bubbling hot tubs or decorative fountains) or when they aspirate (choke on) drinking water that contains the bacteria. Legionellosis cases most often occur in the summer and early fall, but cases can occur at any time of year.