Legionella pneumophila is among the top three or four microbial causes of c
ommunity-acquired pneumonia, yet is often misdiagnosed and inadequately tre
ated. New laboratory tests should simplify the diagnosis. Also, contrary to
common perception, the disease is usually spread via aspiration of water f
rom contaminated hot water distribution systems, not from air conditioning
systems. The treatment of choice has shifted from erythromycin to the newer
macrolides and quinolones. Routine culturing of the hospital water supply
is a requisite first step in preventing hospital-acquired Legionnaires' dis
ease.