Legionellosis is an infection caused by species of the bacterium Legionella, most notably L. pneumophila.
At least 46 species and 70 serogroups have been identified.
L. pneumophila, a ubiquitous aquatic organism that thrives in warm environments (25 to 45 degrees Celsius with an optimum around 35 degrees Celsius) causes over 90 percent of Legionnaires' Disease cases.
An estimated 8,000 to 18,000 people get Legionnaires' disease in the United States each year.
Some people can be infected with the Legionella bacterium and have mild symptoms or no illness at all.