Background

In the summer of 1976, veterans and others who had attended an American Legion convention in Philadelphia began to develop an unidentifiable illness. Many people were hospitalized with pneumonia, fever, and other symptoms. More than 25 died of what was soon termed "Legionnaires' Disease." Eventually, public health officials determined that infection with a certain type ofbacteria was to blame for the illnesses and deaths.^1,2,3,4 Although the bacteria were not named until 1977, species in the genus (a group of related species) Legionella likely also caused previous clusters of pneumonia-like illness before their identification in connection with the 1976 outbreak.

When this page was published, there were at least 50 known Legionella species, 20 of which cause human diseases.^5,6,7 Among Legionellosis cases in the United States, the species Legionella pneumophila causes more than 80 percent of reported infections. L. micdadei, L. longbeachae, L. bozemanii, and L. dumoffii are also frequently associated with Legionellosis cases.

Legionella species grow in a variety of places, including soil and both natural and manmade water sources. In nature, Legionella grow in the thin fresh-water biofilm layer (i.e., slime) on the surface of rivers, lakes, and streams, and rarely causes illness. In manmade settings, Legionella are commonly found in building water systems (e.g., heating, ventilation, and air conditioning; domestic and industrial potable water), hot tubs, and decorative fountains, and are not eliminated by normal chlorination processes (see the General Considerations section on the Control and Prevention page for more information about chlorination).