Pneumonia: Causes, symptoms and treatment of the serious infection
HilaryClinton has been struck down with the infection in recent days
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 eight in every 1,000 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.