Legionellosis is a serious pneumonia caused by bacteria of the genus Legionella assigned to the family Legionellaceae. This family now includes 48 species and over 60 serogroups. Approximately 20 species are implicated in human disease. The overwhelming majority of Legionella infections are caused by Legionella pneumophila. Legionnaires’ disease is the major clinical manifestation of Legionella infection although extra-pulmonary infection and non-pneumonic disease like Pontiac fever occur.