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How dangerous are legionella?

Legionella in drinking water is not uncommon. Basically, bacteria in low concentrations are inevitably an integral part of any freshwater. However, Legionella can cause flu-like illness in humans. But what are legionella bacteria anyway? How are they transmitted and what danger do they actually pose? All this and how you can protect yourself from infection with legionella, you will learn in this article.

Legionella in drinking water is not uncommon. Basically, bacteria in low concentrations are inevitably an integral part of any freshwater. However, Legionella can cause flu-like illness in humans.

But what are legionella bacteria anyway? How are they transmitted and what danger do they actually pose? All this and how you can protect yourself from infection with legionella, you will learn in this article.

What are legionella?

Legionella are rod-shaped bacteria naturally present in surface water and groundwater. Even if the bacteria are always present in the water in small numbers, they may well pose a danger to humans.

Thus, the pathogens cause a wide variety of illnesses, from flu-like symptoms to mild Pontiac fever to serious Legionnaires’ disease in the form of life-threatening pneumonia.

The risk of contracting so-called legionellosis from the pathogen Legionella pneumophila is particularly high when the temperature of the drinking water is optimal for the growth of the bacteria. Thus, temperatures between 25 and 45 degrees provide the optimal conditions for the rapid multiplication of legionella. Only at a hot water temperature of over 60 degrees do the bacteria slowly die. Even at low temperatures below 20 degrees, Legionella hardly multiply, but remain alive.

In addition, the biofilm that develops in the water pipes provides the ideal breeding ground for Legionella. The deposits and coatings in the pipes in combination with the optimal water temperature are therefore a real source of danger of contracting legionella.

How are legionella transmitted?

The pathogens are usually transmitted through the water vapor produced during showering. The pathogen-containing water droplets in atomized or nebulized water enter the lungs of humans through inhalation, where they can cause infection.

The most common sources of Legionella infection include showers, hot tubs, faucets, air conditioners, and humidifiers. In addition, Legionella may be present in other water systems or swimming pools and thus cause different clinical pictures of a Legionella infection.

However, according to experts, drinking water contaminated with Legionella should not usually lead to infection. Accordingly, human health should only be in danger when the pathogens enter the lungs directly .

Pay attention to the right temperature

The disease-causing bacteria Legionella pneumophila feel most at home in pleasantly warm fresh water. Because the optimal temperature and living conditions for the pathogens are found in artificial water systems such as indoor water pipes, most Legionella infections occur at home.

For example, both stagnant water and too low a hot water temperature over too long a period of time can cause a low concentration of Legionella in drinking water to become a serious health hazard.

For this reason, you should always pay attention to the correct temperature in the water tank. Legionella are only slowly killed at temperatures above 60 degrees . From 70 degrees hot water begins the so-called thermal disinfection, which leads to a complete death of harmful bacteria.

Even homeowners who want to save energy due to rising heating and water costs should still make sure that the hot water temperature is at least 60 degrees . Even in the pipes, the temperature should ideally never fall below 55 degrees, otherwise the proliferation of legionella is favored. Even better is to install an automatic legionella circuit. This heats the water in the reservoir to 70 degrees once a week, killing any pathogens and preventing an outbreak of Legionella.

If, despite all precautions, symptoms of legionellosis still occur, it is advisable to consult your family doctor immediately. He or she may be able to refer the patient to a specialist so that the infection can be treated appropriately.

Symptoms of infection

An infection with Legionella can not only cause a wide variety of clinical pictures, but also produce different symptoms depending on the type of disease.

As soon as the pathogens infect the lungs, particularly severe cases are referred to as Legionella pneumonia. This is the well-known Legionnaires’ disease, which manifests itself in the form of severe pneumonia. As a rule, the following symptoms appear in sufferers within a very short time:

  • general malaise
  • severe cough
  • Headache
  • Chest pain
  • Chills
  • high fever of more than 40 degrees

In contrast, Pontiac fever, which is caused by Legionella, is somewhat milder. This disease is also usually characterized by flu-like symptoms:

  • general malaise
  • Headache
  • Limb pain
  • Fever
  • Chest pain
  • Cough

In some cases, Legionella infection can also cause gastrointestinal symptoms such as diarrhea and vomiting, and urinary tract infections .

How dangerous are legionella?

The illnesses caused by Legionella usually manifest themselves through a so-called pneumonia. In the case of a mild infection caused by inhalation of Legionella bacteria via the water vapor – for example when taking a shower – a kind of febrile flu occurs, which heals on its own after a few days.

Serious pneumonia caused by legionella, on the other hand, can become quite life-threatening. In Germany, about 40,000 to 50,000 people are said to die each year as a result of pneumonia. How many of these deaths are actually due to Legionella infection is unknown. However, some studies show that about 15,000 to 30,000 cases of pneumonia are caused by Legionella . The mortality rate of legionellosis here is said to be between 10 and 15 percent.

Incidentally, Legionnaires’ disease was named after the unexplained deaths of 29 U.S. Army Legionnaires . At a joint meeting, about 180 people present fell ill with serious pneumonia or legionellosis. Since the germs were later discovered in lung tissue, the disease they caused was given the name Legionnaires’ disease.

The severity of the disease ultimately depends to a large extent on the age and state of health of the person affected. Thus, immunocompromised persons, the elderly or even babies are particularly at risk. Nevertheless, the health risk posed by legionella should not be underestimated for other population groups either. Thus, even with appropriate treatment, severe pneumonia may well have sequelae.

For this reason, the legislator requires regular legionella tests, which are equipped with so-called “commercially used water systems”. In addition, an increased concentration of legionella in the drinking water must be reported immediately to the health department, as must a legionella infection itself.


When do legionella bacteria become dangerous?

A low number of legionella in drinking water does not represent a danger. Only when a value of 10,000 germs per 100 milliliters of water is reached is a shower ban initiated in order to contain the Legionella risk.

What do I do if I have legionella in the water?

If a legionella test is positive, this must be reported to the landlord and the health department immediately – within 14 days at the most. In addition, it is essential to comply with any shower ban and to use legionella filters.

How to shower with legionella?

Experts advise to use a legionella filter for protection. In the event of a shower ban, one must either use a legionella filter or switch to traditional washing with a washcloth.

How long to run water legionella?

In order to completely kill legionella using thermal disinfection, the water in the pipes must be heated to at least 70 degrees. This should then be flushed through any water pipes and fittings for at least 3 minutes. CAUTION: This must be performed by qualified personnel.


In principle, low numbers of legionella pose no danger. However, should a legionella infestation be present, this can certainly become a high health risk – especially for immunocompromised, elderly people or even babies and small children.

In order to eliminate the risk of legionella, it is therefore essential to comply with the legal requirements.


Legionella symptoms: recognize and treat the disease

Legionella are rod-shaped bacteria that are a component of fresh water worldwide. Especially in the water pipes of buildings the optimal conditions for the growth of environmental germs prevail. Legionella infection can have far-reaching consequences for human health.

In this article, you will learn how the pathogens enter the human body, which symptoms can be used to detect legionellosis and which treatment methods are possible.

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