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How Legionnaires’ Disease spreads
Legionnaires’ disease is caused by Legionella bacteria, which grow in water systems. People usually become infected by breathing in tiny droplets of contaminated water. It is not normally spread from person to person.
The bacteria can be found in warm, stagnant water where they have had time to multiply. Common sources include cooling towers, hot tubs, showers, taps, and other building water systems. The risk is higher when water is aerosolised into a fine mist.
Common places where infection can happen
In the UK, outbreaks have been linked to large buildings and public water systems. Hotels, hospitals, offices, leisure centres, and care homes can all be affected if water systems are not properly maintained. Older pipework and poorly managed hot water systems can increase the risk.
Domestic homes are less commonly linked to outbreaks, but the risk is not zero. Showers, taps, and garden hoses can create small droplets that may be inhaled. Proper temperature control and regular cleaning help reduce the chance of bacteria building up.
How people are exposed
Infection happens when a person breathes in airborne droplets containing Legionella. This can occur while showering, using a spa pool, or standing near equipment that releases water mist. The bacteria then enter the lungs and can cause serious illness.
People can also be exposed through systems that circulate water, such as air-conditioning cooling towers or decorative fountains. The droplets may travel through the air and be inhaled by people nearby. This is why control measures in buildings are so important.
What does not usually spread the disease
Legionnaires’ disease does not usually spread by drinking contaminated water. The main risk comes from inhaling the bacteria in droplets, rather than swallowing them. It is also not considered contagious in everyday contact.
Casual contact with someone who has the illness does not typically pass it on. This means the focus is usually on identifying and controlling the water source. Public health teams investigate outbreaks to stop further exposure.
Who is most at risk
Anyone can develop Legionnaires’ disease, but some people are more vulnerable. Older adults, smokers, and people with weakened immune systems are at greater risk. Those with long-term conditions such as lung disease may also be more likely to become seriously ill.
People who spend time in settings with complex water systems should be especially aware. Travellers staying in hotels, patients in healthcare settings, and workers in buildings with cooling systems may face higher exposure if controls are poor. Regular maintenance is essential to keep water systems safe.
Frequently Asked Questions
Legionnaires' disease transmission refers to how people are exposed to Legionella bacteria, usually by inhaling contaminated water aerosols or, less commonly, by aspiration of contaminated water. It does not typically spread from person to person.
The main sources of Legionnaires' disease transmission are water systems that can generate aerosols, such as cooling towers, hot tubs, decorative fountains, showers, faucets, and large plumbing systems.
Person-to-person Legionnaires' disease transmission is considered extremely rare and is not the usual way the disease spreads. Most infections come from environmental water sources.
Legionnaires' disease transmission occurs when Legionella bacteria grow in warm water and become aerosolized from plumbing or equipment, allowing people to inhale the contaminated mist.
Drinking contaminated water is not the usual cause of Legionnaires' disease transmission, but aspiration of contaminated water into the lungs can lead to infection.
Legionnaires' disease transmission is more likely when water is warm, stagnant, poorly disinfected, or stored in systems that allow bacteria to multiply and spread as aerosols.
Cooling towers can contribute to Legionnaires' disease transmission by dispersing fine water droplets into the air, which may contain Legionella bacteria if the system is contaminated.
Yes, hot tubs can cause Legionnaires' disease transmission if they are contaminated and generate aerosolized water that is inhaled by nearby people.
Showers and faucets can contribute to Legionnaires' disease transmission if Legionella is present in the plumbing and water droplets are inhaled during use.
People at higher risk after Legionnaires' disease transmission include older adults, smokers, people with weakened immune systems, and those with chronic lung disease.
Yes, Legionnaires' disease transmission can happen in homes if household plumbing, water heaters, showers, humidifiers, or other water devices contain Legionella and produce inhalable mist.
Legionnaires' disease transmission in healthcare facilities can occur when contaminated building water systems expose patients, visitors, or staff to inhaled aerosols.
Legionnaires' disease transmission is not caused by ordinary air conditioning itself, but it can occur from contaminated cooling towers or water-based HVAC components that release aerosolized water.
Both are associated with exposure to Legionella, but Legionnaires' disease transmission leads to pneumonia, while Pontiac fever is a milder flu-like illness from the same environmental exposure.
Legionnaires' disease transmission can be reduced by maintaining proper water temperatures, disinfecting systems, preventing stagnation, cleaning equipment, and using water management plans.
Yes, decorative fountains can cause Legionnaires' disease transmission if they are contaminated and release fine water droplets that people inhale.
Yes, Legionnaires' disease transmission can increase during renovations or plumbing changes if water systems are disturbed, disinfectant levels change, or stagnant water is released.
After Legionnaires' disease transmission, symptoms usually appear within 2 to 10 days, though the exact timing can vary.
After a suspected Legionnaires' disease transmission exposure, the building or water source should be evaluated, affected people should seek medical care if symptoms develop, and public health authorities may need to be notified.
Yes, Legionnaires' disease transmission risk can be assessed by testing water systems for Legionella and by inspecting conditions that support bacterial growth and aerosol formation.
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