Bacteria can be found in shower heads and taps, hot tubs, cooling towers, hot water tanks, decorative fountains or water systems in large buildings.
People breathe small droplets of water that contain bacteria, the CDC says.
Symptoms include coughing, shortness of breath, fever, muscle aches and headaches. Symptoms begin two to 10 days after exposure. The doctor will check for pneumonia and may order a urine or phlegm test to check that the Legionella bacterium has caused a lung infection.
Can I get it from someone else?
"Generally, people do not spread legionnaires' disease to other people," the CDC says. The Mayo Clinic says you can't get it from a person's contact.
How many people die from legionnaires?
About one in every 10 people with Legionnaire's disease dies, according to the CDC. Legionnaires' disease is treated with antibiotics and most people who become ill make a full recovery.
How many cases are there per year?
About 7,500 US cases of Legionnaires' disease were reported to the CDC in 2017, the agency said, adding that it was probably an understatement because the disease was underdiagnosed. The reported percentage of people receiving Legionnaires' disease has increased by 550% since 2000.
There are various factors behind the increasing number of cases, including the real increase in people receiving the disease because of riskier people, but also because of increased reporting.
How to prevent it?
There are no vaccines that can prevent Legionnaires' disease. Instead, according to the CDC, legionnaires' disease can be prevented by ensuring that "building owners and managers maintain water-building systems to reduce the risk of legionella growth and spread."
Who is most at risk?
People 50 or older, current or former smokers, and people with chronic lung disease or a weak immune system. "Avoiding smoking is the most important thing you can do to reduce your risk of infection," says the Mayo Clinic.
Where did the name come from?
There is a conjunction with the American Legion Convention in Philadelphia in 1976. According to the CDC, 29 people have died and 180 have fallen ill from their usual hotel.