Legionnaires' Disease and Pontiac Fever (Legionellosis)

  • Medical Author:
    George Schiffman, MD, FCCP

    Dr. Schiffman received his B.S. degree with High Honors in biology from Hobart College in 1976. He then moved to Chicago where he studied biochemistry at the University of Illinois, Chicago Circle. He attended Rush Medical College where he received his M.D. degree in 1982 and was elected to the Alpha Omega Alpha Medical Honor Society. He completed his Internal Medicine internship and residency at the University of California, Irvine.

  • Medical Editor: William C. Shiel Jr., MD, FACP, FACR
    William C. Shiel Jr., MD, FACP, FACR

    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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Where is the Legionella bacterium found?

Legionella organisms can be found in many types of water systems. However, the bacteria reproduce to high numbers in warm, stagnant water (95 F-115 F), such as that which is found in certain plumbing systems and hot-water tanks, cooling towers, and evaporative condensers of large air-conditioning systems, and whirlpool spas. Cases of legionellosis have been identified throughout the United States and in several other countries. The disease likely occurs worldwide.

The bacterium thrives in the mist sprayed from air-conditioning ducts. Thus, it can infest an entire building or airplane. Factors that are now known to enhance the growth of Legionella bacteria in manmade water environments include the following:

  • Water temperatures of 77 F-107.6 F
  • Stagnation of the water
  • Scale and sediment in the water
  • Certain free-living amoebae organisms in water capable of supporting intracellular growth of legionellae

How can Legionnaires' disease be prevented?

Prevention is aimed at improving the design and maintenance of cooling towers and plumbing systems. The goal is limiting the growth and aerosolization of Legionella organisms. Hyperchlorination tends to be ineffective at disinfecting the water supply. Copper-silver ionization, super heating, and monochloramine disinfection are newer methods of controlling and eliminating growth of this bacteria.

During outbreaks, health investigators seek to identify the source of disease transmission and recommend appropriate prevention and control measures, such as decontamination of the water source.

Detailed guidelines for the prevention of Legionnaires' disease both in hospitals with no identified cases ("primary prevention") and in hospitals with reported cases ("secondary prevention") have been published and are available in the U.S. from the Centers for Disease Control and Prevention.

Medically reviewed by Robert Cox, MD; American Board of Internal Medicine with subspecialty in Infectious Disease


Flanery, B., et al. "Reducing Legionella Colonization in Water Systems With Monochloramine." Emerg Infect Dis 12 (2006): 588.

Neil, K., et al. Increasing Incidence of Legionellosis in the United States, 1990-2005; Changing Epidemiologic Trends. Clin Infect Dis 47 (2008): 591.

Von Baum, Heike, et al. "Community Acquired Legionella Pneumonia; New Insights From the German Competence Network for Community Acquired Pneumonia." Clin Infect Dis 46 (2008): 1356.

Yu, V.L., et al. "Livofloxacin Efficacy in the Treatment of Community Acquired Legionellosis." Chest 125 (2004): 2135.

Medically Reviewed by a Doctor on 3/23/2016

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