[Legionellosis: its suspicion imposes a diagnostic confirmation and a probabilistic treatment]

Legionellosis. Leginellosis is an important cause of community- acquired and hospital-acquired pneumonia. In France and in Europe, Legionella pneumophila serogroup 1(Lp1) is the causative agent in most cases. The incidence of the disease is underestimated. The main risk factors are age, underlying debilitating conditions, and immunosuppression. This is an "atypical" pneumonia that might clinically ressemble to other bacterial pneumonias and is often severe enough to require hospital admission. Urinary antigen detection is the first-line diagnostic test, although it is limited to Lp1. Sample culture of the lower respiratory tract is recommended since it enables the diagnosis of all legionella spp and outbreak investigation. Disease notification is required. Macrolides (azithromycin) and fluoroquinolones (levofloxacin) are effective against L. pneumophila. β- lactams and aminoglycosides are ineffective.