Pneumonia is often caused by a bacterial infection and results in swelling of the tissues in one or both lungs.
It can become serious within hours and people suspected of pneumonia should see a doctor.
Symptoms include a cough, which may produce mucus or be dry, and people may have difficulty breathing or feel short of breath.
They may also have a rapid heartbeat, fever, sweats, chills, and chest pain.
Less common symptoms include coughing up blood, headache, extreme fatigue, nausea, and vomiting.
In the UK, pneumonia is thought to affect around eight in 1,000 adults a year, according to the NHS Choices website.
It is most common during the winter months and although it can affect people of any age, it is more common and often more severe in older or very young people.
People with health conditions such as asthma, cystic fibrosis, heart, kidney or liver disease may also be more affected alongside people with weakened immune systems, such as cancer patients.
Pneumonia can be difficult to diagnose because the symptoms are sometimes confused with the common cold, bronchitis and asthma.
Mild pneumonia is usually best treated at home with plenty of rest, a course of antibiotics, and plenty of fluids.
More serious cases will need to be treated in hospital. Symptoms – such as fatigue – may persist for several months.
Possible complications of pneumonia include pleurisy, where the walls between the lungs and chest become inflamed, lung abscess, or blood poisoning.
Pneumonia is usually not contagious, so it is safe for patients to be around other people.