Several outbreaks of legionellosis  have been described in North America, Asia and Europe.

The incidence of this disease is not known because not all cases are diagnosed.

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The genus Legionella comprises around 60 species, each representing dozens of serogroups. But the species Legionella pneumophila alone is responsible for more than 90% of cases of legionellosis diagnosed by culture, and more than 85% of cases are caused by isolates from serogroup 1. The diagnosis of non-serogroup 1 L. pneumophila cases is on the rise, however, because of the more widespread use of PCR as a diagnostic method. The only exceptions are Australia and New Zealand, where Legionella pneumophila serogroup 1 strains are responsible for just 45.7% of cases of legionellosis and Legionella longbeachae is associated with 30.4% of cases.

Legionella bacteria are part of the aquatic flora and are common in warm freshwater environments. The presence in water systems of organic deposits and other microorganisms, as well as iron, zinc and aluminum, encourages bacterial growth. The bacteria are resistant to heat and can therefore be found at the bottom of hot water tanks. They are intracellular bacteria but they can survive outside cells. In water environments they multiply in amoebae, then when the amoebae die they spread through the water and are ingested by a new host (a cell), enabling new cycles of multiplication.


Legionellosis (the most common and severe form of which is known as Legionnaires' disease) acquired its name after an outbreak in 1976 affecting 182 delegates at the 58th National Convention of the American Legion in Philadelphia, 29 of whom died. The cause was found to be Legionella pneumophila bacteria, which thrive in warm water (30-60°C) and had spread through the air conditioning system in their hotel. The recent emergence of legionellosis can be attributed to its affinity for modern water supply systems such as cooling towers, air-conditioning systems, spa pools, hot tubs and hot water pipes.

Frequent outbreaks

Since 1976, several outbreaks of legionellosis have been described in North America, Asia and Europe. It is thought that there are between 8,000 and 18,000 cases of legionellosis every year in the United States. But the actual incidence is not known because not all cases are diagnosed and/or reported. In France, between 1,600 and 2,000 cases are reported each year. 

Improved surveillance has led to more effective detection of clustered outbreaks, and since 1998 several outbreaks linked to cooling towers have been identified. The outbreak that occurred in winter 2003 in the Pas-de-Calais in France was the largest observed in France to date, in terms of both the number of cases (nearly 90 recorded cases and 17 deaths) and the size of the affected area – cases of infection were found around 10 kilometers away from the identified source of the outbreak, a cooling tower in the town of Harnes. This outbreak particularly highlighted how difficult it is to control the spread of Legionella pneumophila outbreaks because for the first time the industrial source needed to be shut down for full decontamination on two occasions, a month apart, to curb the outbreak.


The recognized transmission route in humans is the inhalation of infectious aerosols from aquatic environments. Once the aerosols have been inhaled, the bacteria are absorbed through the pulmonary alveoli before invading immune system cells known as macrophages, which they eventually destroy. Only one case of human-to-human transmission has been reported in the literature. The case involved very close proximity between the two people in a confined space for a long time. People with legionellosis are not contagious.

Showers, spa pools and cooling towers are the main causes of contamination, but any contaminated water that generates aerosols should be considered as a potential source of contamination. 

Symptoms and treatment

After an incubation period of 2 to 10 days, legionellosis causes acute pulmonary infection with pneumonia-like symptoms. The first signs resemble influenza (fever, dry cough). The fever then worsens, with temperatures reaching as high as 39.5°C, and patients experience a general feeling of fatigue and weakness, as well as abdominal pain (nausea, vomiting, diarrhea) and neurological symptoms (from confusion and impaired consciousness to coma).

Legionellosis can give rise to complications in two areas: irreversible respiratory failure and acute kidney failure, both of which often prove fatal. Legionellosis is more common in over-50s and in men; other risk factors are smoking, diabetes, cancer or any blood disease, and treatment with corticosteroids or immunosuppressants.

The bacteria are naturally resistant to the types of penicillin usually used to treat lung conditions, but they can be effectively treated with other antibiotics, such as macrolides (azithromycin or erythromycin), fluoroquinolones or rifampicin, if prescribed early enough.

At the Institut Pasteur

The Biology of Intracellular Bacteria Unit, led by Carmen Buchrieser, specializes in Legionella research. It carries out comparative and functional genomic and molecular epidemiology studies to understand how Legionella hijacks host functions to cause disease. Recent research has revealed a mechanism used by Legionella pneumophila to "reprogram" gene expression in the cells it infects, facilitating the survival and proliferation of Legionella pneumophila during infection. The unit has also shed light on the evolutionary mechanisms that may enable bacteria that are harmless in the natural environment to become dangerous pathogens for humans.

January 2021

The Institut Pasteur teams working on the topic

Research units

Biology of Intracellular Bacteria laboratory led by Carmen Buchrieser

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