In our review article on Legionella, we will discuss the history and circumstances of the bacterium's discovery, common locations, how it is dangerous, who is at risk of infection and what defenses can prevent it from showing up in your water heater.

What is Legionella?

Legionella is a genus of bacteria that pose a pathogenic danger to humans. These are small microorganisms with sizes from 0.2 to 0.7 microns in diameter and from 2 to 20 microns in length that live in fresh water. They cause the formation of fever and pneumonia with possible negative health consequences. Today, dozens of strains and serotypes of the bacterium are known, differing in the degree of activity and requiring a different approach to treatment. Most healthy people with strong immune systems are not affected by Legionella, but people with weakened immunity should beware of it. The bacterium does not pose an epidemiologic danger, because the disease is not transmitted from person to person, but is a factor of external environmental exposure. Legionella likes warmth and humidity, it reproduces very quickly under these conditions.

History of the discovery of bacteria


In 1976, Legionella came into serious discussion following a convention of American Legion veterans held in Philadelphia on the eve of the United States' 200th anniversary. Subsequent to the congress, a group of delegates was diagnosed with an outbreak of an unidentified disease. Pneumonia was the primary symptom observed among the 221 rally participants hospitalized, leading to the unfortunate loss of 32 lives. This event garnered significant media coverage and sparked understandable concern among the public, prompting thorough investigation into the incident and its underlying causes. Research revealed a substantial colony of unidentified pathogenic bacteria in the ventilation system of the congress hall, responsible for the outbreak. With many elderly veterans possessing compromised immune systems, they were particularly vulnerable to the pathogen, resulting in fatal consequences for individuals already burdened with various underlying health conditions. The newly discovered bacteria were consequently named "Legionella" in remembrance of the tragic event preceding their identification by scientists.

It should be said that similar disease outbreaks have been observed before: in 1957, 1959, 1965, 1968 and 1974, but their pathogen was never detected. But after 1976, when the event received a wide resonance in society, the problem returned and required active actions. Today, many varieties of Legionella, places and conditions of its localization have been identified and studied, and control measures and effective treatment methods have been developed.

Water heaters with anti-legionella system

Legionnaires' disease and risk factors


The disease caused by legionella bacteria is called Legionnaires' disease or legionellosis. It manifests itself in the form of symptoms similar to the flu or pneumonia: high temperature, fever, cough. Myalgia, headache, diarrhea and confusion are sometimes observed. Symptoms appear about a week after infection, and the disease itself lasts several weeks until complete recovery. Severe cases of the disease are rare, but with a weakened immune system and lack of effective treatment, death is possible.


Therefore, in the high-risk group, those who are most susceptible to the disease, are people who find it more difficult to resist any infections:

  • elderly people, especially after 65 years;
  • heavy smokers of any age;
  • suffering from chronic lung pathologies;
  • people with weak immunity.

The latter category includes cancer patients, diabetics, people suffering from cardiovascular diseases, dialysis patients, AIDS patients, as well as people who abuse alcohol and take medications that weaken the immune system.


The problems of proper treatment of Legionnaires' disease are related to the difficulty of unambiguous diagnosis due to the blurriness of the clinical symptoms of the disease and the inability to clearly distinguish between atypical and bacterial forms of pneumonia. With accurate diagnosis, like most bacterial infections, Legionnaires' disease is cured with narrow-spectrum antibiotics.

Where can Legionella be found?

The bacterium is found everywhere, from natural bodies of water to home hot water supply systems. It reproduces very well in conditions of temperature 35 – 40 ° C and pH = 6.9, that is, in warm acidified water. It can be found in contaminated hand showers, bathtubs, Jacuzzis, spas, fountains, swimming pools, car washes, storage water heaters, humidification and irrigation systems, wet pipeline fittings, air conditioning and ventilation systems.

In our country, the issue of Legionella within the hot water supply system is not currently a pressing concern, and it is hoped that it will remain so in the foreseeable future. This can be attributed to the widespread use of centralized hot water supply systems, which boast substantial water flow rates and maintain high temperatures of 60 – 65°C within pipelines. These conditions are not conducive to the rapid proliferation of Legionella bacteria. Conversely, autonomous hot water supply systems in private residences, often using storage tanks, pose a potential risk for the dissemination of pathogenic microorganisms. To address the prevention of Legionella proliferation from other sources, the installation of specialized antibacterial filters offers an effective solution.

Water heaters with a maximum water temperature of 80 °C

Legionella in the water heater

High adaptability allows Legionella to successfully colonize artificial water reservoirs such as storage water heater tanks, which become a source of infection for various reasons. The first reason is to purchase a water heater with an excess storage volume that exceeds the family's needs for hot water. Because of this, unused water remains in the tank, which stagnates, becoming a breeding ground for bacteria. Stagnant water with a colony of reproduced bacteria emits an unpleasant odor or, simply, a stench. In the same way, infrequent use of the water heater and neglect of regular maintenance of the internal tank have a bad effect on the possibility of bacterial population growth. It must be cleaned, disinfected and the magnesium anode (if any) replaced annually. Removing scale, dirt and disinfecting the tank using a citric acid solution will not only extend the life of the water heater, but will also guarantee your health.


An additional complexity of preventive actions in the fight against pathogens in places of their possible dislocation is that bacteria often parasitize in the body of amoebae, serving them not only as a habitat and reproduction, but also as an effective buffer against chemical disinfection (chlorine) and ultraviolet radiation. In addition, amoebae, hiding in their gut insidious bacteria, do not allow them to be detected in laboratory water analysis.

Bacterial protection system in storage water heater

The system of bacterial protection of the water heater against the growth of pathogenic bacteria in the water tank is based on research on the conditions of Legionella. According to published data from the World Health Organization, the survival of Legionella bacteria is decisively influenced by the ambient temperature:

  • 20 °C and below — is in a resting state
  • 25 – 45 °C – temperature range when growth occurs
  • 32 – 42 °C is the perfect temperature range for growth
  • 48 – 50 °C – live bacteria stop reproducing
  • 50 °C – depending on the strain, 90% of bacteria die within 1.5 – 2 hours
  • 60 °C – 90% of bacteria die within half an hour
  • 65 °C — kills 90% of bacteria within 2 minutes
  • 70 – 80 °C – the entire colony dies almost instantly

Thus, we can conclude that the best way to combat pathogenic microflora in storage water heaters is to heat the water in the tank to temperatures of 60 – 65 °C, and preferably 70 – 80 °C. This is exactly what we are talking about when you read about the special equipment of the water heater model with the “anti-legionella” function.


The name of the “anti-legionella” function comes down to the simplified activation of the tank heating mode at maximum temperature by pressing one button. But even if this function is not reflected in the device’s operating manual, you can sanitize the water in the tank by simply turning the thermostat knob to the appropriate value on the temperature scale. Those, you need to manually set the temperature to maximum and start heating the tank. The anti-legionella system destroys active microorganisms in water and prevents their further reproduction. In order to forget about this problem forever, you should periodically carry out hygienic heat treatment of the tank.

Manufacturers provide another option to combat bacteria, which is based on the antibacterial properties of the inner coating of the water tank. Models from a number of well-known brands are equipped with enameled or glass-ceramic tanks, the surface composition of which contains active silver, which eliminates the bacterial threat. But the best option would be to use a water heater with an expensive copper tank with high antibacterial potential. Copper, even without extreme heat, effectively kills 99.9% of various bacteria, including Legionella, E. coli and staph.

A good preventive method to keep the water in the tank clean is the use of bacterial filters installed on the supply to the appliance. In particularly neglected cases, for 100% guarantee of disinfection and death of bacteria, a chemical method of tank cleaning should be used, consisting in acid flushing with a special pump.

Water heaters with magnesium anode

Conclusions and recommendations

Summarizing everything we know about Legionella now, you can significantly reduce the risk of bacteria developing in your storage water heater and avoid possible infection by following the recommendations below:

  • do not buy or use a water heater of excessive volume, which can cause water stagnation and the development of pathogenic flora;
  • purchase water heaters with an upper heating threshold of at least 65 °C ;
  • follow the manufacturer's recommendations for keeping the internal tank of the water heater clean, performing its maintenance in a timely manner;
  • install antibacterial filters at the entrance to the water heater;
  • try to maintain the water temperature in the water heater at least 55 °C;
  • If possible, buy and use a water heater with an internal copper tank.

To be honest, the anti-legionella function is nothing more than ordinary marketing. After all, almost any water heater can heat water to a temperature that destroys bacteria. But doing this with one click on a function button or turning the thermostat to maximum is your choice. In our opinion, it is not worth overpaying for the convenience of activating this function imposed by the manufacturer. It will be enough to choose a simple model with a high maximum temperature, which will get rid of Legionella.