Pontiac fever: A flu-like illness caused by the bacterium Legionella pneumophila contracted by breathing mist that comes from a water source (such as air conditioning cooling towers, whirlpool spas, showers) contaminated with the bacteria. The incubation period is short, from a few hours to 2 days, before the onset of fever and muscle aches. Persons with Pontiac fever do not have pneumonia. They generally recover in 2 to 5 days without treatment. Pontiac fever is so-named because of an outbreak in 1968 in Pontiac, Michigan. It is a milder form of legionellosis than Legionnaire disease which is caused by the same bacterium.