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Legionnaires' Disease

Legionellosis / Legionella / Legionnaires' Disease / Legion Fever

Legionellosis is a potentially fatal infectious disease caused by gram negative, aerobic bacteria belonging to the genus Legionella. Legionnaires’ disease is caught by inhaling small droplets of contaminated water. Outbreaks are rare as it is not contagious and cannot be spread directly from person to person.

Legionella bacteria is commonly found in sources of water, such as rivers and lakes. The bacteria sometimes find their way into artificial water supply systems such as:

  • Air conditioning systems
  • Hot and cold water services
  • Cooling towers
  • Baths and showers
  • Fountains
  • Sprinkler systems
  • Whirlpool baths
  • Spas
  • Humidifiers that were being used in food display cabinets

Given the right conditions, the bacteria can rapidly spread and contaminate these water systems. The 2 things this virus requires to grow and reproduce are:

  • Water temperature of between 20-45ºC (68-113ºF)
  • Impurities in the water that the bacteria can use for food, such as rust, sludge, algae and limescale

Large buildings, such as hotels, hospitals, museums and office blocks, are more vulnerable to Legionella contamination because they have larger, more complex water supply systems where the contamination can quickly spread.


Legionnaire’s disease can be very serious particularly in the elderly or people with a pre-existing health condition. An estimated 10-15% of otherwise healthy people who contract Legionnaires’ disease will die.

It is common for the symptoms of Legionnaires’ disease to begin to show between 6 to 7 days. However, symptoms can begin any time from 2 to 19 days after exposure to the initial infection. For the first stage of infection you will experience the following symptoms:

  • Mild headaches
  • Muscle pain

This is followed by the onset of more severe symptoms, including:

  • High fever, sometimes a temperature of 38ºC (100.4ºF) or above
  • More severe muscle pain
  • Chills
  • Tiredness
  • Changes to your mental state, such as confusion

Once the bacterium begins to infect the lungs, you may also experience:

  • A persistent cough, which is usually dry at first but as the infection develops you may start
  • Coughing up mucus or, rarely, blood
  • Shortness of breath
  • Chest pains

Some symptoms may also affect the digestive system causing:

  • Feeling sick
  • Being sick
  • Diarrhoea
  • Loss of appetite


Legionnaires’ disease is treated with antibiotics to kill the bacteria. Possible antibiotics that may be used include Erythromycin or Clarithromycin. These may be taken orally in tablet or capsules form. Alternatively they may be given through an intravenous infusion where the medicine is given in hospital by a continuous drip.

Depending on the severity of the condition, it will usually need to take antibiotics for 7 to 10 days, although in some cases they may need to be taken for up to 3 weeks. In particularly severe cases of Legionnaires’ disease, a number of life-threatening complications can occur:

  • The lungs are unable to provide the body with enough oxygen
  • The kidneys do not work, which can lead to a dangerous build-up of fluids and waste in the blood
  • A blood infection, leading to a sudden and dangerous drop in blood pressure

Find out more about Infection Control

This website contains general information about medical conditions and treatments. The information is not advice, and should not be treated as such. Please read our medical disclaimer.