Legionella bacteria cultured in a laboratory
Legionella is a bacterial disease, and the bacteria can be cultured in laboratories for diagnostic purposes.

Legionella disease

Legionella disease, legionellosis, is a lung disease caused by the bacteria Legionella.

The Legionella bacteria is common in nature, but in low concentrations. Its growth is enhanced by ideal temperature conditions which are between 20 – 50 degrees Celsius, a pH of 6-7 and low concentration of salts. The most important infection sites are cooling towers, shower facilities, bubble baths, air conditioning and ventilation systems. It is an airborne infection, transferred by aerosols from water, and can of course also be transferred from person to person. The disease acquired its name after an outbreak of a disease in 1977. It was called a “mystery disease” at first. The disease involved 221 persons and caused 34 deaths. The outbreak was first noticed among people attending a convention of the American Legion, an association of US military veterans, thereof the name “Legionella”.

Cooling towers Cooling towers are a common source of Legionella outbreaks. © Colourbox

Disease description

The Legionella disease varies in severity from a mild febrile illness to a serious and sometimes fatal form of pneumonia and is caused by exposure to Legionella species found in water. Worldwide, waterborne Legionella pneumophila is the most common cause of cases including outbreaks. Legionella pneumophila and related species are commonly found in lakes, rivers, creeks, hot springs and other bodies of water. Other species including L. longbeachae.

Legionellosis is a generic term describing the pneumonic and non-pneumonic forms of infection.

a) The non-pneumonic form (Pontiac disease) is an acute, self-limiting influenza-like illness usually lasting 2–5 days. The incubation period is from a few and up to 48 hours. The main symptoms are fever, chills, headache, malaise and muscle pain (myalgia). No deaths are associated with this type of infection.

b) The Legionnella can also give lead to the development of pneumonia, which is a very serious type of this infection. Death occurs through progressive pneumonia with respiratory failure and/or shock and multi-organ failure.

The Legionella disease has an incubation period of 2 to 10 days up to 16 days has been recorded in some recent outbreaks).

The disease may be life threatening if not treated properly with antibiotics.


The incidence of community-acquired Legionnaires’ disease varies widely according to the level of surveillance and reporting. Since many countries lack appropriate methods of diagnosing the infection or sufficient surveillance systems, the rate of occurrence is unknown. In Europe, Australia and the USA there are about 10–15 cases detected per million persons. Of the reported cases 75–80% are over 50 years and 60–70% are male. Other risk factors for community-acquired and travel-associated legionellosis include: smoking, a history of heavy drinking, pulmonary-related illness, immuno-suppression, and chronic respiratory or renal illnesses.


Control of Legionella growth can occur through chemical or thermal methods. Some countries have specific environmental regulations mentioning how to prevent the Legionella problem.

  • Warm water should have a temperature of 70 degrees of Celsius and there should be routines for flushing the pipe systems regularly
  • Once a year shower heads etc. should be disinfected
  • Companies using cooling towers should have technical measures that prevent microbiological growth and proper maintenance and control routines
  • Companies with air conditioning and ventilation systems must be aware of the problem and control and clean the relevant pipes and equipment regularly

Suggested reading for interested: Fact sheet no. Legionellosis from WHO, 2014

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This article is from the free online course:

Occupational Health in Developing Countries

University of Bergen