It's been in and out of the news recently, as famous faces have revealed how they've thought to be suffering from pneumonia.
Hillary Clinton, Niall Horan and now Jeremy Clarkson have all said to have suffered from the illness.
The presenter is said to have taken 'indefinite leave' after being taken ill while on holiday in Majorca.
But what is pneumonia? And what are the symptoms?
Pneumonia is often caused by a bacterial infection and leads to swelling of the tissue in one or both lungs.
It can become serious within a matter of hours and people with suspected pneumonia should seek medical attention.
Symptoms include a cough, which can produce mucus or may be dry, and people may have difficulty breathing or feel short of breath.
They may also have a rapid heartbeat, fever, sweating, shivering and chest pain.
Less common symptoms include coughing up blood, headaches, extreme tiredness, nausea and vomiting.
In the UK, pneumonia is thought to affect around 1 in 125 UK adults each year adults per year, according to the NHS Choices website.
It is more common in the winter months and, while it can affect people of any age, it is more common and often more serious in the elderly or very young.
Those with health issues such as asthma, cystic fibrosis, heart, kidney or liver disease can also be more affected alongside people with a compromised immune system, such as cancer patients.
Pneumonia can be difficult to diagnose because symptoms are sometimes confused with colds, bronchitis and asthma.
Mild pneumonia is usually best treated at home by taking plenty of rest, a course of antibiotics and plenty of fluids.
More severe cases will need to be treated in hospital. Symptoms - such as fatigue - can linger for several months.
Possible complications of pneumonia include pleurisy, where the linings between the lungs and ribcage become inflamed, a lung abscess or blood poisoning.
Pneumonia is not usually contagious, and so it is safe for patients to be around other people.
Boots UK is piloting a new pneumonia vaccination service to help raise the health and happiness of the nation by providing adults 50 years and over, who are more likely to catch pneumonia than those aged under 50, with an option to help protect themselves.
Boots pharmacist, Angela Chalmers comments: “Many people don’t realise that anyone can catch pneumonia, not just those deemed at ‘high risk’. Additionally, flu can increase chances of catching pneumonia. Specially trained pharmacists in Boots stores offering this service are able to provide the pneumonia vaccination service and flu vaccination service and can also offer additional advice on staying well this winter.”
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