{{Automatic taxobox
| image = Legionella_Plate_01.png
| image_caption = Legionella sp. under UV light
| taxon = Legionella
| authority = Brenner et al. 1979
| subdivision_ranks = Species
| subdivision =
Legionella adelaidensis
Legionella anisa
Legionella beliardensis
Legionella birminghamensis
Legionella bozemanae
Legionella brunensis
Legionella busanensis
Legionella cardiaca
Legionella cherrii
Legionella cincinnatiensis
Legionella clemsonensis
Legionella donaldsonii
Legionella drancourtii
Legionella dresdenensis 
Legionella drozanskii
Legionella dumoffii
Legionella erythra
Legionella fairfieldensis
Legionella fallonii
Legionella feeleii
Legionella geestiana
Legionella genomospecies 1
Legionella gormanii
Legionella gratiana
Legionella gresilensis
Legionella hackeliae
Legionella impletisoli
Legionella israelensis
Legionella jamestowniensis
Candidatus Legionella jeoniiLegionella jordanisLegionella lansingensisLegionella londiniensisLegionella longbeachaeLegionella lyticaLegionella maceacherniiLegionella massiliensis Legionella micdadeiLegionella monrovica Legionella moravicaLegionella nagasakiensisLegionella nautarumLegionella norrlandicaLegionella oakridgensisLegionella parisiensisLegionella pittsburghensisLegionella pneumophilaLegionella quateirensisLegionella quinlivaniiLegionella rowbothamiiLegionella rubrilucensLegionella sainthelensiLegionella santicrucisLegionella shakespeareiLegionella spiritensisLegionella steelei Legionella steigerwaltiiLegionella saoudiensis Legionella taurinensisLegionella thermalisLegionella tucsonensisLegionella tunisiensisLegionella wadsworthiiLegionella waltersiiLegionella worsleiensisLegionella yabuuchiae}}
The genus Legionella is a pathogenic group of Gram-negative bacteria. It includes the species causing a pneumonia-type illness called Legionnaires' disease, and a mild flu-like illness called Pontiac fever.

It may be seen with a silver stain. Legionella is common in many environments, including soil and aquatic systems, with at least 50 species and 70 strains identified.

The side-chains of the cell wall carry the bases responsible for the somatic antigen specificity of these organisms. The chemical composition of these side chains both with respect to components as well as arrangement of the different sugars determines the nature of the somatic or O antigen determinants, which are essential means of serologically classifying many Gram-negative bacteria.Legionella got its name after a 1976  outbreak of a then-unknown "mystery disease" sickened. 221 people were infected, with 34 deaths. The outbreak was first noticed among people attending a convention of the American Legion—an association of U.S. military veterans.

On January 18, 1977, the cause was identified as a previously unknown bacterium later named Legionella.

Legionnaire's disease and Pontiac fever
Legionnaires' disease is a severe form of pneumonia—lung inflammation usually caused by infection. Legionnaires' disease is caused by a bacterium known as Legionella.

People can't catch Legionnaires' disease from person-to-person contact. Instead, most people get Legionnaires' disease from inhaling the bacteria. Older adults, smokers and people with weakened immune systems are particularly susceptible to Legionnaires' disease.

The Legionella'' bacterium also causes Pontiac fever, a milder illness resembling the flu. Separately or together, the two illnesses are sometimes called legionellosis. Pontiac fever usually clears on its own, but untreated Legionnaires' disease can be fatal. Although prompt treatment with antibiotics usually cures Legionnaires' disease, some people continue to experience problems after treatment.

Disinfectants help people to avoid catching it.

References 

Gram-negative bacteria