What is legionnaires disease?

Probably not: Legionnaires' disease is a severe form of pneumonia — lung inflammation usually caused by infection. Legionnaires' disease is caused by a bacterium known as legionella. You can't catch legionnaires' disease from person-to-person contact. Instead, most people get legionnaires' disease from inhaling the bacteria. Older adults, smokers and people with weakened immune systems are particularly suscep.