Legionnaires’ disease is a serious lung infection caused by Legionella bacteria.

Initial symptoms usually include flu-like symptoms, such as:

• mild headaches
• muscle pain
• high temperature (fever), usually 38C (100.4F) or above 
• chills
• tiredness
• changes to your mental state, such as confusion

Once bacteria begin to infect your lungs, you may also experience symptoms of pneumonia, such as:

• a persistent cough – which is usually dry at first, but as the infection develops you may start coughing up phlegm or, rarely, blood
• shortness of breath 
• chest pains

See your GP as soon as possible if you develop the symptoms above. If you have a high temperature, you may have an infection of some kind. Seek urgent medical attention if you have more severe symptoms, such as chest pain and breathing difficulties.

It usually takes six to seven days between getting the infection and the start of symptoms (known as the incubation period), although it can be any time from two to 19 days.

As Legionnaires' disease has similar symptoms to other illnesses, your recent travel history will be relevant in making a diagnosis. If an infection is suspected, a urine test can be used to check for Legionnaires' disease. 

How does Legionnaires’ disease spread?

Legionnaires’ disease is caused by Legionella bacteria infecting your lungs. It's usually caught by breathing in small droplets of contaminated water. The infection isn't contagious and can't be spread directly from person to person.

Legionella bacteria is usually found (often in harmlessly low numbers) in sources of water, such as ponds, rivers and lakes. However, the bacteria can rapidly multiply if they find their way into artificial water supply systems, such as air conditioning systems.

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 in which the bacteria can quickly spread.

The two things that Legionella bacteria need to grow and reproduce are:

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

Although rare, Legionnaires' disease has also come from contaminated showers, sprinkler systems and spas.

Legionnaires' disease is rare in the UK. In 2013, 284 people were reported to have the infection in England and Wales. Of these cases, 88 people (31%) were exposed to the infection while travelling abroad – mainly to Mediterranean countries, but also tropical countries such as India. However, given the millions of trips made abroad each year, 88 cases is a very small number.

Cases of Legionnaires' disease arising in England and Wales usually peak between July and September.

Increased risk

Everyone is potentially at risk of developing Legionnaires’ disease. However, certain things make it more likely that you will experience a more severe form of the infection. These include:

• being 50 years of age or over – 235 (83%) of the 284 confirmed cases in 2013 involved people over 50 years of age
• smoking, or having smoked heavily in the past (a recent study has shown that smoking cannabis may also increase your risk)
• drinking alcohol heavily 
• about three-quarters have an underlying medical condition, such as diabetes, kidney disease, or a pre-existing lung condition
• having a weakened immune system – for example, people with HIV and AIDS or cancer

Treating Legionnaires' disease

Legionnaires' disease is treated with a course of antibiotics, such as erythromycin or clarithromycin. Antibiotics will be taken as a tablet or capsule, or through a continuous drip into a vein in your arm.

How long you'll need antibiotic treatment for will depend on how severe the condition is. Treatment usually lasts about a week, but may continue for up to three weeks.

Around 90% of people with Legionnaires' disease make a full recovery after taking antibiotics. However, while you may start to feel better after a few days, it can take weeks until you're completely back to normal. During this period, it's common to feel tired. 

Legionnaires' disease can be particularly serious in people with pre-existing health conditions. In this case, you may be admitted to hospital so your health can be closely monitored.

Complications of Legionnaires' disease

In some cases, Legionnaires' disease can lead to further, life-threatening problems.

Severe infection can cause some organs, such as your lungs or kidneys, to stop working properly. Another complication is septic shock. An estimated 10% of otherwise healthy people who develop Legionnaires' disease die due to problems like these.

During 2011 to 2013, there were 84 deaths from Legionnaires’ disease in England and Wales (including overseas travellers). Most deaths occur in people who are 70 years of age or older.

Preventing Legionnaires' disease

The best way to prevent an outbreak of Legionnaires’ disease is to ensure that any water system under your control is properly maintained and conforms to relevant health and safety regulations.

This mainly involves keeping water either cooled below 20C or heated above 60C. The water supply should also be kept free of impurities and moving, so that it doesn't stagnate.

The Health and Safety Executive (HSE) is an independent regulator for reducing work-related death and serious injury in Britain. It provides guidance about Legionnaires' disease on its website, including information and advice about managing legionella in hot and cold water systems.

If you're travelling abroad, be aware of the risk of Legionnaires’ disease and seek medical advice if you develop any symptoms of the infection. There's currently no vaccine that offers protection against Legionnaires' disease.

If you're at increased risk of developing severe Legionnaires’ disease – for example, because you have a weakened immune system, you may want to consider avoiding water systems that could be contaminated, such as spas.