Legionella is a bacterium, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophila. Often associated with wet damp areas in specific temperature ranges within air conditioning heating and ventilation systems.
Legionella is common in many environments, with at least 50 species and 70 serogrops identified. Legionella acquired its name after a July, 1976 outbreak of a then-unknown "mystery disease" sickened 221 persons, causing 34 deaths. The outbreak was first noticed among people attending a convention of the American Legion – a congressionally chartered association of U.S. military veterans. The convention in question occurred in Philadelphia during the U.S. Bicentennial year. This epidemic among U.S. war veterans, occurring in the same city as – and within days of the 200th anniversary of – the signing of the Declaration of Independence, was widely publicized and caused great concern in the United States. On January 18, 1977 the causative agent was identified as a previously unknown bacterium, subsequently named Legionella.
Legionella live within amoebae in the natural environment. Legionella species are the causative agent of the human Legionnaires' disease and the lesser form, Pontiac fever. Legionella transmission is via aerosols — the inhalation of mist droplets containing the bacteria. Common sources include cooling towers, swimming pools, home hot-water systems, fountains, and similar sources that tap into a public water supply. Natural sources of Legionella include freshwater ponds and creeks.
Once inside a host, incubation may take up to two weeks. Initial symptoms are flu-like, including fever, chills, and dry cough. Advanced stages of the disease cause problems with the gastrointestinal tract and the nervous system and lead to diarrhea and nausea. Other advanced symptoms of pneumonia may also present.
However, the disease is generally not a threat to most healthy individuals, and tends to lead to harmful symptoms only in those with a compromised immune system and the elderly. It should b checked for wherever there is long standing water basements and air-conditioning and heating systems.
In the United States, the disease affects between 8,000 to 18,000 individuals a year.