Legionellaspp. are significant causes of both community-acquired pneumonia and nosocomial pneumonia. More than 40 species ofLegionellahave now been identified. The spectrum of disease ranges from asymptomatic infection to serious disease, with two specific syndromes identified: Legionnaire's disease and Pontiac fever. Hospital-acquired infection arises from the presence ofLegionellain the hospital water supply. The optimal approach for the detection and prevention of nosocomial infection is debatable-whether or not periodic sampling of hospital water systems should be carried out in the absence of clinical cases is controversial. Newer macrolides or newer fluoroquinolone agents are the preferred therapy for serious diseases caused byLegionella.