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American Legion

What is Legionnaires' disease?

Liz Szabo
USA TODAY

Legionnaires' disease is a severe form of bacterial pneumonia. It's caused by the bacteria legionella, which gets its name from an outbreak that killed 29 people attending an American Legion at a Philadelphia hotel in 1976.

Legionella bacteria are found naturally in the environment, usually in water, according to the Centers for Disease Control and Prevention. The bacteria grow best in warm water, such as water in hot tubs, cooling towers, air conditioners, mist sprayers in grocery stores, hot water tanks, large plumbing systems and decorative fountains. Legionella do not seem to grow in car or window air conditioners.

This image depicts a large grouping of Gram-negative Legionella pneumophila bacteria.

People get Legionnaires' disease when they breathe in a mist or vapor containing the bacteria, such as the water droplets sprayed from a hot tub that hasn't been properly cleaned, according to the CDC. The bacteria don't spread from person to person, like the flu or a cold.

Most people exposed to legionella don't become sick. Smokers, the elderly and those with weakened immune systems are more susceptible than others to Legionnaires' disease, according to the Mayo Clinic. Symptoms include fever, chillls, headaches and muscle pain, which can progress to coughing, chest pain, nausea, vomiting and difficulty breathing.

Between 5% and 30% of patients with Legionnaires' die, according to the CDC.

Legionella also can cause a milder disease, called Pontiac fever, whose symptoms are similar to the flu. Together, Legionnaires' disease and Pontiac fever are called legionellosis, according to the Mayo Clinic.

While Pontiac fever usually goes away on its own, Legionnaires' disease can be fatal and requires prompt treatment with antibiotics.

There were 2,472 cases of legionellosis in the first seven months of 2015, as well as 2,270 in the first seven months of 2014, according to the CDC.

Cases have been increasing in recent years.

There may be several reasons for the increase, said Anne Schuchat, director of CDC's National Center for Immunization and Respiratory Diseases. First, tests for legionellosis are more readily available, making it easier to get an official diagnosis.

Second, the population of vulnerable people has grown. There are more elderly people today, as well as people with weakened immune systems, she said.

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