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Legionnaires' disease


Disease: Legionnaires' disease Legionnaires' disease
Category: Respiratory diseases

Disease Definition:

Caused by a bacterium known as legionella, Legionnaires' disease is a severe form of pneumonia.


Inhaling the bacteria causes this disease. People can't catch Legionnaires' disease from person-to-person contact. People who are especially susceptible to this disease include smokers, older adults and people with weakened immune systems.


Pontiac fever, which is a milder illness that resembles the flu, is also caused by the legionella bacterium. Separately or together, the two illnesses are sometimes called legionellosis. In most cases, Pontiac fever clears on its own. However, when Legionnaires' disease is left untreated, it could be fatal. Prompt treatment with antibiotics usually cures Legionnaires' disease, however, after the treatment, some people continue experiencing problems.

Work Group:

Symptoms, Causes


Usually, after being exposed to the legionella bacteria 2 to 14 days, Legionnaires' disease occurs. Its initial signs and symptoms usually include:


  • Chills
  • Headache
  • Fever that could be 40 C (104 F) or higher
  • Muscle pain


By the second or third day of developing Legionnaires' disease a person may experience other signs and symptoms, such as:


  • Fatigue
  • Cough, which could bring out mucus, and in some cases even blood
  • Loss of appetite
  • Confusion or other mental changes
  • Shortness of breath
  • Gastrointestinal symptoms, such as diarrhea, nausea and vomiting
  • Chest pain


Mostly, this disease affects the lungs. However, in some cases, it could also cause infections in wounds and in other parts of the body, such as the heart.



The mild form of Legionnaires' disease is called Pontiac fever. Some of its characteristics are:


  • Headache
  • Fever
  • Muscle aches
  • Chills


In case someone has Pontiac fever, their symptoms will clear in about two to five days. This form of the disease doesn't infect the lungs.


Pontiac fever affects people with no known risk factors for the illness, and it has a short incubation period, usually two to three days, unlike Legionnaires' disease. Some researchers believe that Pontiac fever isn't an infection at all, but a hypersensitivity reaction to legionella bacteria or to the single-celled organisms (protozoa) in which they replicate.


In case someone thinks that they’ve been exposed to legionella bacteria, they should see a doctor. In case he/she has taken trips in the past two weeks, they should mention them to the doctor, along with where they’ve stayed. To help shorten the recovery period and prevent serious complications, Legionnaires' disease should be diagnosed and treated as soon as possible. Additionally, prompt treatment is vital for people at high risk.


Although several species of legionella could cause infection, but the legionella pneumophila bacterium is the cause of most cases of Legionnaires' disease. Legionella bacteria thrive in warm, damp environments. In nature, they could survive for months in soil, lakes, hot springs and rivers. However, the natural environment rarely causes problems because the levels of bacteria are so low. The biggest threat lies indoors, where legionella bacteria could multiply in water systems, air conditioning systems, whirlpool spas and even the misters in grocery store produce departments.


The legionella bacteria have the ability to anchor themselves to the interiors of pipes, faucets and shower heads, just like many microorganisms. Once attached, the bacteria replicate in a sticky substance called a biofilm. The water dislodges some of the biofilm as it flows past, and disperses bacteria throughout the water system. A person could contract Legionnaires' disease from home plumbing systems; however, most of the outbreaks have occurred in large buildings, which could be because complex systems allow the growth and spread of the bacteria more easily.



When people inhale microscopic water droplets that contain legionella bacteria, they become infected. This could be water dispersed through the ventilation system in a large building, or it could be the spray from a shower, faucet or whirlpool. Several sources have been associated with outbreaks, some of them are:


  • Physical therapy equipment
  • Whirlpool spas on cruise ships
  • Water systems in hotels, nursing homes and hospitals
  • Swimming pools
  • Cooling towers in air conditioning systems
  • Decorative fountains


Exactly how much exposure to the bacteria is needed to cause the disease is still not known. However, some people after inhaling contaminated droplets for just a few minutes have developed infections. The legionella bacteria could be capable of traveling as far as four miles through the air, unlike many bacteria, which spread within a small radius.


Despite the fact that this bacteria usually spread through aerosolized water droplets, but the infection could also be transmitted in other ways, such as:


After working in the garden or using contaminated potting soil, some people have contracted Legionnaires' disease. In case earth that contains the bacteria is stirred up at large construction sites, the disease could also spread.


If liquids accidentally enter someone’s lungs when they cough or choke while drinking, this type of transmission occurs. Someone may develop this disease if they aspirate water that contains legionella bacteria.



Immune system cells (alveolar macrophages) that usually attack and destroy foreign organisms will surround legionella bacteria once they enter someone’s lungs. However, legionella bacteria turn these cells to their advantage, rather than being destroyed by them. The legionella bacteria enter the macrophages, use them to grow and replicate, and then kill them. Thousands of new bacteria become released into the lungs once the macrophages die. This worsens the symptoms during the first week of the infection and perpetuates the cycle of the disease.


Not everyone who is exposed to legionella bacteria becomes sick. people will be more susceptible to the infection in case they:


  • Smoke: Smoking makes people more susceptible to all types of lung infections because it damages the lungs.
  • Have a chronic lung disease, such as emphysema or other serious conditions such as cancer, kidney disease or diabetes.
  • Have a weakened immune system due to HIV/AIDS or certain medications, particularly drugs that are taken after a transplant to prevent organ rejection or corticosteroids.
  • Have a job maintaining the cooling towers in air conditioned systems
  • Are 65 or older


In hospitals and nursing homes, where the germs could spread easily and people are vulnerable to infection, Legionnaires' disease is a sporadic and local problem.



Various life-threatening complications could be caused by Legionnaires' disease, such as:

Acute kidney failure:

When this condition occurs, the kidneys suddenly lose their ability to perform their main function, which is eliminating excess fluid and waste material from the blood. Dangerous levels of fluid and waste accumulate in the body when the kidneys lose their filtering ability.

Septic shock:

When a severe and sudden drop in blood pressure reduces blood flow to vital organs, particularly the kidneys and brain, this complication occurs. By increasing the volume of blood pumped, the heart will try to compensate for this. However, the heart will weaken and the blood flow will be reduced even further due to the extra workload.

Respiratory failure:

When the lungs can't remove enough carbon dioxide from the blood, or when they are no longer able to provide the body with enough oxygen, this complication occurs.


Legionnaires' disease could be fatal if it's not treated promptly and effectively, particularly if the person’s immune system is weakened by medications or disease.


Pontiac fever doesn't cause any lingering problems and usually goes away on its own. Antibiotics are used in treating Legionnaires' disease. To reduce someone’s chances of complications or death, treatment should start early.


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