(from the Oxford English Dictionary)
Legionnaires' disease: a severe form of bacterial pneumonia (often accompanied by mental confusion) which is caused by Legionella pneumophila and is associated esp. with infected water systems. [So called from the outbreak in July 1976 that affected people attending a Legionnaires' Convention in Philadelphia.]
1976 Kingston Whig-Standard (Ontario) 5 Aug. 1/4 Further clues to the flu-like ‘legionnaires [sic] disease’ may come today when first results are available from tests being conducted at the state health laboratories in Philadelphia. 1977 Lancet 17 Dec. 1266/1 It is already important for clinicians to think of legionnaires' disease when severe pneumonia proves resistant to standard therapy. 1978 G. VIDAL Kalki vii. 173 Every bright-faced child on earth is scheduled to die sooner or later, of cancer, legionnaire's disease, swine flu, whatever. 1983 Oxf. Textbk. Med. I. V. 332/2 Two cases of Legionnaires' disease in an Oxford transplant unit were associated with infected shower water. Ibid., It has been suggested that Legionella pneumonia be the term used to describe pneumonia due to Legionella species and reserve Legionnaires' disease for outbreaks similar to that in Philadelphia. 1985 Sci. Amer. Oct. 70/3 It is a DNA probe designed to recognize the presence in a patient of any one of the 22 species of the bacterial genus Legionella, the agents of various pneumonias of which the best-known is Legionnaires' disease.