How Does A Person Get Legionnaires Disease?

Legionnaires’ disease is a form of atypical pneumonia originated by any type of Legionella bacteria. Within the legionella there are several species, from which the most important is Legionella pneumophila, since it is the one that causes most cases of pneumonia in humans.

Legionella pneumophila is a microbe that usually lives in water. It has been isolated in natural sources of fresh water, in lakes, in thermal waters, etc. But these natural sources cause legionnaires disease very rarely. The vast majority of cases and epidemics are associated with artificial sources of water: water pipes in buildings, heat exchangers, cooling towers and cooling pipes for example.

How Does A Person Get Legionnaires Disease?

The transmission of the legionnaires disease is by inhalation of water particles contaminated by L. pneumophila. That is, from those places where there is L. pneumophila, aerosols are produced and in the droplets of that aerosol “legionella” travels and enters the lung of the future patient when inhales air containing these particles. The environment in which the inhalation of L. pneumophila occurs is usually in those buildings or facilities with contaminated devices (cooling towers), and people living in the vicinity of those facilities can also be affected. Another way to contract the legionnaires disease is also by aspiration of aerosols that can be generated in the vicinity of works in which large amounts of soil are removed. The way to inhale from drinking water pipes is through the sprays that are generated when the faucets are opened, especially the showers. However, the legionnaires disease has not been linked to water intake and neither has person-to-person transmission been demonstrated.

In this way, pneumonia due to L. pneumophila can appear in an isolated and occasional manner as one of the possible causes of pneumonia acquired in the community and also in an epidemic form, usually associated with specific buildings. If one of these buildings is a hospital, the consequences can be especially serious since people exposed to L. pneumophila will be mostly patients with other diseases that will make more difficult for them to fight against the infection.

The main known risk factors for a patient to contract pneumonia due to Legionella are smoking and chronic obstructive pulmonary disease. There are also other risk factors that lead to legionnaires disease:

-Immunosuppressive treatments, especially with corticosteroids.

-Surgical patients.

-Alcoholism, malnutrition.

-Receptors for organ transplantation.

Legionella can cause several clinical symptoms, of which the most important is pneumonia, which is what we know as “Legionnaires’ disease”. To refer to the set of diseases that it can cause, we use the term “Legionellosis.”

The incubation period for legionnaires disease is between 2 and 10 days, and the onset of symptoms is usually gradual with symptoms reminiscent of flu: malaise, myalgias, anorexia, and headache that are then followed by the rest of the symptoms. The fever is usually very high and can exceed 40 degrees C. However, the legionnaires disease symptoms can also have a sudden onset and from the first moment have a serious situation with very high fever and great impairment of the general condition. There is usually a cough with poor sputum production and there may be very intense chest pain. In 15 to 50% of the cases there are also digestive symptoms, of which the most frequent is usually diarrhea.

Regarding analytical data, several alterations have been described, of which perhaps the most striking is the presence of hyponatremia (low levels of sodium in the blood). However, none of them allows establishing the diagnosis.

Clinical suspicion of legionnaires disease is established when:

-Persistent fever>40 ºC.

-No germs on sputum examination under a microscope (the well-known sputum Gram stain).

-Hyponatremia (low blood sodium level).

-Failure to treat with some antibiotics such as penicillins, cephalosporins and aminoglycosides.

-Coincidence with possible sources suspected of being the origin of the infection.


After the Legionella grows and multiplies in the water system of a building, the water that contains the Legionella must be dispersed in the form of droplets small enough for people to inhale. People can get legionnaires disease when they inhale droplets of water in the air that contain bacteria.

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