Legionnaires’ disease: it’s rare, but probably not quite as rare as you might think.

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This form of pneumonia was responsible for 345 confirmed cases in England and Wales in 2016, and between January-March 2017 there were 32 definite Legionnaires’ outbreaks, although cases in the UK usually peak between July and September.

And more recently, there has been an outbreak in Majorca, affecting British holidaymakers.

But what exactly is this disease that most people have heard of but often don’t know much about? Here’s what you need to know:

1. Legionnaires’ disease is a severe type of pneumonia, an infection that causes inflammation of the air sacs in the lungs. The sacs fill with liquid, making it difficult for the lungs to transport oxygen around the body in the blood.

2. Legionnaires’ is caused by the legionella bacterium, which infects people when they inhale tiny droplets from a contaminated source.

3. People infected with the legionella bacteria typically become ill two to 14 days after exposure. Early symptoms are flu-like and include muscle aches, tiredness, headaches, dry cough, chest pains, shortness of breath, confusion and fever.

4. In some people, usually those with weakened immune systems, the disease can be fatal. Most deaths occur in people who are aged 70 or older.

5. Legionnaires' can lead to life-threatening complications including organ failure and septic shock. An estimated 10% of otherwise healthy people who develop Legionnaires' disease die due to such complications

6. Legionella bacteria grows in warm water, usually between 20-45C. It can be found in harmlessly low numbers in ponds, rivers and lakes, but will multiply rapidly in artificial water supply systems such as air conditioning systems.

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7. The bacteria is only a risk to health when the water temperature allows it to multiply rapidly, often in water systems which aren’t properly designed, installed and/or maintained.

8. Large buildings such as hotels, hospitals 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.

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

10. The disease is treated with antibiotics, usually for about three weeks. Around 90% of people with Legionnaires’ make a full recovery after taking antibiotics.

11. There’s a national surveillance scheme for Legionnaires’ disease to detect clusters and outbreaks of legionella in England and Wales, and identify and control sources of infection. It’s a notifiable disease, meaning health professionals must inform local health protection teams of suspected cases.

12. Legionnaires’ disease gets its name from members of the American Legion who caught it while attending a convention at a hotel in Philadelphia in 1976.

13. People of all ages can get Legionnaires’ disease, but it mainly affects people over 50. It hits three times more men than women, and smokers, heavy drinkers and people with a weakened immune system are at higher risk.

14. Legionnaires’ disease isn’t contagious and can’t be spread from person to person.

15. Around a third of UK cases are exposed to the infection while abroad, mainly in Mediterranean countries, but also in tropical countries such as India.