Legionella sp. bacterium, computer artwork. This Gram-negative flagellated bacterium is the cause of legionellosis, or Legionnaires' disease. The bacterium is naturally found in water, and can inhabit reservoirs of water inside buildings, such as in air conditioning systems. The first recorded outbreak of the disease was at an American Legion convention in Philadelphia, USA, in 1976. 34 people died in the outbreak and over 200 required medical treatment. It is from this outbreak that the bacterium takes its name. Infection causes pneumonia and fever, although prompt treatment with antibiotics can effect a complete cure.