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Legionella bacteria can cause two illnesses which may be considered legionellosis: Legionnaires' disease, a kind of pneumonia (lung disease), and a milder infection without pneumonia, known as Pontiac fever. These bacteria are found naturally in the environment, usually in wet places like natural bodies of water and moist soil, as well as in man-made water handling systems. The bacteria grow best in warm water like the kind found in hot tubs, cooling towers, hot water tanks, large plumbing systems, and parts of the air-conditioning systems of large buildings. People get legionellosis when they breathe in a mist or vapor (small droplets of water in the air) that has been contaminated with the bacteria. Any misting device which is not properly maintained and cleaned can harbor Legionella, including grocery store produce misters, outdoor mist tents, car washes and decorative fountains.

The bacteria are NOT spread from one person to another person. When multiple people become ill in the same place at about the same time, such as patients in hospitals, people on cruise ships, or people attending a convention at a large hotel, it is due to widespread exposure to contaminated water — not person-to-person contact.

Signs of Legionnaire's disease can include a high fever, chills, and a cough. Some people may also suffer from muscle aches and headaches. These symptoms usually begin two to 10 days after being exposed to the bacteria. Legionnaires' disease can be very serious and can cause death. The elderly, those with respiratory conditions, and those with compromised immune systems are most at risk. However, most cases can be treated successfully with antibiotics, and healthy people usually recover from infection.

Pontiac fever may also include fever, headaches, and muscle aches, but tends to have a shorter duration — usually two to five days — and symptoms do not include pneumonia. Usually Pontiac fever will resolve on its own without treatment and without further complications.

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