A genus of bacteria of the family Legionellaceae. It currently consists of at least 51 serogroups comprising 34 species. It has the ability to colonize water in distribution systems (heating tanks, cooling towers, air conditioning lines, etc). It can cause disease in humans (e.g., Legionnaires disease or Legionellosis) that is progressive and sometimes fatal, or a milder form of pneumonic illness (Pontiac fever) that is self-limited (i.e., heals on its own) with respiratory symptoms similar to influenza.

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