Legionellosis is a bacterial disease which may cause pneumonia.
Most cases occur as single isolated events.
Outbreaks are relatively rare. Subsequently, the bacterium causing the illness was named Legionella pneumophila and the name of the illness was changed to legionellosis. While the bacterium was only recently identified, earlier cases have been confirmed as far back as 1947, and cases probably occurred before that date. It is estimated that about 25,000 people develop legionellosis in the United States each year. An additional unknown number are infected with the legionella bacterium and have mild symptoms or no illness at all.