What is Legionnaires' Disease?

Legionnaires' disease is a serious and potentially life-threatening form of pneumonia caused by breathing in water droplets contaminated with Legionella bacteria. It's named after an outbreak at an American Legion convention in 1976, where the disease was first identified.

While not everyone exposed to Legionella bacteria will become ill, the disease can be severe, particularly for vulnerable individuals.

How Do You Catch Legionnaires' Disease?

Legionnaires' disease is contracted by inhaling tiny water droplets (aerosols) that contain Legionella bacteria. Common sources include:

  • Showers and taps
  • Spa pools and hot tubs
  • Cooling towers and air conditioning systems
  • Fountains and water features
  • Humidifiers and misters
  • Car washes

Important: Legionnaires' disease is not spread from person to person. You cannot catch it by drinking contaminated water or through casual contact with an infected person.

Symptoms of Legionnaires' Disease

Symptoms typically appear 2 to 10 days after exposure and can include:

  • High fever (often above 39°C)
  • Persistent cough (may produce mucus or blood)
  • Shortness of breath
  • Chest pain
  • Headache and muscle aches
  • Confusion or changes in mental awareness
  • Nausea, vomiting, or diarrhoea
  • Extreme tiredness

If you experience these symptoms after exposure to water systems, seek medical attention immediately and inform your doctor about possible Legionella exposure.

Who Is Most at Risk?

While anyone can develop Legionnaires' disease, certain groups face higher risk:

  • People aged 50 and over
  • Smokers and former smokers
  • People with chronic lung conditions (COPD, asthma, emphysema)
  • Those with weakened immune systems (due to illness, medication, or chemotherapy)
  • People with chronic health conditions (diabetes, kidney disease, heart disease)
  • Heavy drinkers

For these groups, exposure to even low levels of Legionella can pose serious health risks.

Treatment and Recovery

Legionnaires' disease is treated with antibiotics, usually administered in hospital. Early diagnosis and treatment significantly improve outcomes.

Recovery time varies:

  • Most people recover fully with prompt treatment
  • Hospital stays typically last 7–10 days
  • Full recovery can take several weeks or months
  • Some people may experience long-term fatigue or reduced lung function

Mortality rate: Even with treatment, Legionnaires' disease can be fatal in approximately 10% of cases, rising to 25% or higher in vulnerable populations.

Prevention: The Importance of Water Testing

The best way to prevent Legionnaires' disease is to control Legionella bacteria in water systems before they pose a risk.

Regular water testing helps:

  • Identify contamination early
  • Verify that control measures are working
  • Demonstrate compliance with health and safety duties
  • Protect building occupants and visitors
  • Provide documented reassurance

If you're responsible for a building with water systems—whether a care home, rental property, hotel, school, or workplace—routine Legionella testing is a sensible precaution.

Learn more about Legionella bacteria
Find out who needs Legionella testing
Order Legionella test kits

Pontiac Fever: A Milder Form

Legionella bacteria can also cause Pontiac fever, a milder illness with flu-like symptoms that usually resolves without treatment within 2–5 days. Unlike Legionnaires' disease, Pontiac fever does not cause pneumonia and is rarely serious.

UK Reporting and Outbreaks

In the UK, Legionnaires' disease is a notifiable disease, meaning doctors must report cases to public health authorities. The UK Health Security Agency (UKHSA) monitors outbreaks and provides guidance on prevention and control.

Between 300–500 cases are reported in England and Wales each year, though many cases are contracted abroad during travel.

Key Takeaways

  • Legionnaires' disease is a serious lung infection caused by Legionella bacteria
  • It's contracted by breathing in contaminated water droplets, not person-to-person
  • Symptoms include high fever, cough, and breathing difficulties
  • Vulnerable groups face higher risk and more severe outcomes
  • Early treatment with antibiotics is essential
  • Prevention through water system management and testing is the best defence

If you have questions about Legionella risk in your building or need help with water testing, get in touch with our team.