An environmental bacterium that inhabits soil and fresh water in the natural world. In general, this bacterium reproduces between 20°C and 50°C, with 36°C being ideal. In populated areas, Legionella bacteria are mainly found in water, such as in the cooling systems of air conditioners and in circulating bathtubs of bathing facilities. Legionella pneumonia caused by a Legionella infection can be severe, with some individuals dying within seven days of onset.

Legionella bacteria cannot be transmitted from person to person, and infection is usually through direct inhalation of an aerosol containing the bacteria (such as water mist).

Controlling the quality of water that may be a source of infection is necessary to prevent infection.

Several effective Legionella bacteria disinfection methods are currently used. Chlorine dioxide disinfection is considered an effective method in bathing facilities, a source of several cases of infection in Japan.