Legionellosis refers to the two clinical syndromes caused by Legionella : Pontiac fever, a benign febrile illness and Legionnaires'disease (or pneumonia). Clinically and radiologically, Legionnaires'disease presents itself as a « typical » pneumonia caused by Streptococcus pneumonia. Diagnosis is usually made by using urinary antigen testing. Culture and PCR are also helpful. Legionella is resistant to betalactam antibiotics, and is treated by quinolones or macrolides.