Legionnaires disease (Legionellosis) fact sheet

Legionnaires' disease (Legionellosis) fact sheet Legionnaires' disease (Legionellosis) is a serious and sometimes fatal form of pneumonia caused by the bacteria Legionella. Although not all cases of Legionnaires' disease are severe, up to ten per cent of cases can be fatal. There are over forty strains of Legionella bacteria but only a few cause disease in humans. The strains that are most commonly associated with human disease are pneumophila and longbeachae.