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.
Cases occur sporadically and in outbreaks.
Outbreaks occur most often in the summer but cases occur all year round. Legionellosis can be a mild respiratory illness or it can be severe enough to cause death. From 10 to 40 percent of healthy adults have antibodies showing previous exposure to the organism, but only a small percentage have a history of previous pneumonia. Legionella are widely distributed in our environment. They have been found in creeks and ponds, hot and cold water taps, hot water tanks, water in air conditioning cooling towers and evaporative condensers, and soil at excavation sites.