Legionellosis is a serious lung infection caused by bacteria of the genus Legionella. The bacteria mainly occur in aqueous environments and can be transmitted to humans via aerosols in the air.

What legionellosis?

Legionellosis is an infection of the lung caused by a bacteria of the genus Legionella. In more than 90% of cases in Belgium, legionellosis is caused by the species L. pneumophila. Legionella bacteria are inhaled through the air and can infect the lungs in this way.


The symptoms of legionellosis appear 2 to 10 days (though in some cases also up to 20 days) after infection:

  • (high) fever
  • chills
  • muscle pain, headache
  • shortness of breath and/or cough
  • chest pain
  • confusion
  • sometimes vomiting and diarrhoea.

The disease can take a very serious course and possibly also lead to the death of the patient. An infection with Legionella cannot be distinguished clinically from pneumonia, for which a microbiological examination is required.


The Legionella bacterium is found mainly in aqueous environments and in moist soil. The bacterium grows in stagnant water with a temperature of 20-50°C, such as cooling towers, showers, humidifiers, etc. The bacterium can survive below this temperature, but not multiply. Legionella bacteria can end up in the air via aerosols (small water droplets) and, therefore, be inhaled by people through the air.


The bacterium grows in water with a temperature of 20 to 50° Celsius. A lower temperature prevents the bacteria from multiplying and a higher temperature kills the bacterium. These principles can be applied for the prevention of infection with this bacterium.

Installations containing water, such as sanitary installations, cooling towers and certain medical devices, that have not been used for a long time pose a risk of infection. Necessary precautions need to be taken when restarting or reusing such equipment.


The disease is clinically indistinguishable from pneumonia, and a further test must be conducted for this:

  • antigen detection in urine

  • culture of bacteria based on sputum, pleural fluid, lung tissue…
  • PCR test.

Risk groups

  • Travelers.
  • The proximity or use of systems that produce aerosols (small water droplets).
  • Certain occupations increase the probability due to the proximity or use of systems or standing water (e.g. cooling tower or swimming pool staff, cleaners, healthcare personnel, etc.).


Legionellosis is treated with antibiotics. In some cases, hospitalisation is also required. After all, the disease can take a serious course.


Sciensano contributes to the epidemiological surveillance of legionellosis in Belgium. For this, we work together with the relevant laboratories and the regional authorities.

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