Legionellosis, commonly known as Legionnaires disease, is a potentially fatal respiratory disease caused by Legionella pneumophila, a gram negative, aerobic bacteria that is carried by amoeba that thrive in stagnant water. During infection, the bacterium invades macrophages and lung epithelial cells, and replicates intracellularly. It belongs to the genus Legionella.

It is not transmitted from person to person. It is acquired through inhalation of aerosolized water contaminated with the bacteria. Sources where temperatures allow the bacteria to thrive include hot water tanks, cooling towers and evaporative condensers of large air-conditioning systems such as those commonly found in hotels and large office buildings. Though the first known outbreak was in Philadelphia, Pennsylvania, cases of legionellosis have occurred throughout the world.

Legionellosis takes two distinct forms:

  • Legionella, is the more severe form of the infection and produces high fever and pneumonia. Legion fever requires treatment with antibiotics and aggressive pulmonary management to resolve. Depending on age and medical condition, Legion fever has between a 5% and 30% mortality rate.
  • Pontiac fever is the milder form of the infection that produces an upper respiratory infection without pneumonia. It resembles acute influenza.  Pontiac fever is not known to be fatal. It also has a spontaneous resolution.