Informal an acute respiratory infection, often with pneumonia, caused by bacteria (Legionella pneumophila) that may contaminate water or soil
See Legionnaires' disease in American Heritage Dictionary 4
(lēˌjə-nârzˈ)
noun
An acute, sometimes fatal respiratory disease caused by a bacterium of the genus Legionella, especially L. pneumophila, and characterized by severe pneumonia, headache, and a dry cough.