Legionnaires' disease - Causes of Legionnaires' disease

Legionnaires' disease is caused by an infection of legionella bacteria in your lungs.

You can become infected with legionella bacteria by inhaling droplets of contaminated water in the air, or in rare cases from water that goes down the "wrong way" as you drink.

You cannot develop Legionnaires' disease from simply drinking contaminated water and it does not spread between people.

Legionella bacteria

Legionella bacteria can be found in any freshwater environment - such as rivers and lakes - although it is incredibly rare to become infected from these sources. This is because the bacteria are often only present in low numbers as the temperature of the water is usually too low for the bacteria to grow and spread.

However, if the bacteria manage to find their way into an artificial water system, given the right circumstances, they can quickly grow and reproduce, leading to a widespread contamination of the water system.

The two things that the legionella bacteria require to grow and reproduce are:

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

Water systems known to be vulnerable to legionella contamination include:

• hot and cold water systems for large buildings, such as hotels and hospitals
• air conditioning systems that use water for cooling purposes
• cooling towers

However, any artificial water system is potentially vulnerable to contamination. Although rare in the UK, cases of Legionnaires’ disease have reportedly arisen from contaminated:

• baths and showers
• fountains
• sprinkler systems
• whirlpool baths
• spas
• humidifiers that were being used in food display cabinets

Read about preventing legionnaires' disease for information on how to reduce the risk of water contamination.

Increased risk

Everyone is potentially at risk of developing Legionnaires’ disease. However, a number of things make it more likely for certain individuals to experience a more severe form of the infection, including:

• being 50 years of age or over
• being a tobacco smoker or having a past history of heavy smoking, although a recent study has shown smoking cannabis may also increase your risk
• being a heavy drinker
• having diabetes
• having kidney disease
• having a weakened immune system (the body’s natural defence against infection and illness) – for example, people with HIV and AIDS 
• having cancer, particularly lung cancer or leukaemia (cancer of the white blood cells)
• having a pre-existing lung condition, such as chronic obstructive pulmonary disease (COPD)
• having a heart condition, such as heart failure  
• having a liver condition, such as alcoholic liver disease