Chest infections are very common in the elderly and often follow a cold or flu.

They need careful monitoring in people in their 80s as they can be serious, but the Duke of Edinburgh is a remarkably fit man with no known complications, such as asthma or chronic obstructive pulmonary disease, so should make a swift recovery.

Prince Philip, 86, did not attend a thanksgiving service for Sir Edmund Hillary, the man who first climbed Everest, on Wednesday because he had been taken ill with a cold. The chest infection has followed.

Often doctors cannot determine the actual cause of a chest infection, but viruses account for most cases of bronchitis and bacteria for many cases of pneumonia. Patients are often given antibiotics as an insurance against bacterial pneumonia even if the cause has not been identified.

Dr Keith Prowse, chairman of the British Lung Foundation, said: “The doctors will perhaps want to carry out an X-ray of the chest to ensure there’s no indication of pneumonia or damage to the wall of the chest,” he said.

“If he is coughing anything up then they will want to send it for culture analysis in the lab to find out what kind of infection it is.

“Quite often, antibiotics are started before the results come back. Because of this, he will be treated with a broad-spectrum antibiotic that kills a lot of things.

“Antibiotics should ensure that any yellow or green phlegm turns to a clear colour as treatment progresses.”

Doctors will also want to check the Duke’s white blood cell count to see how the body is responding to infection, Dr Prowse said.

Depending on the count of different types of white blood cell, experts can glean whether it is a bacterial or viral infection.

Dr Prowse said: “In older people you would also want to make sure they were not developing a fever or any other kind of complication.

“In older people, because their resistance is less than younger people, then the risk of it progressing to say pneumonia is much greater.

“But if it’s caught early then he should be alright.

“Most people with a chest infection are treated at home but if they are elderly they may be admitted to hospital to be safe.

“People can be admitted because they are either very ill, have an odd chest infection - such as something they may have caught abroad - or are showing signs of complications like having a high fever.

“In many circumstances the majority of people with a chest infection respond well to a course of antibiotics.”

Of the two most common chest infections, bronchitis occurs in 44 per 1,000 people per year, usually during autumn and winter, and pneumonia in 5-11 per 1,000. Pneumonia is responsible for more than 80,000 hospital admissions a year and is the fifth leading cause of death.

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