The Queen has been forced to withdraw from her engagements and is resting at home after being diagnosed with a chest infection.
Chest infections – an infection of the lungs or large airways – are common, especially after a cold or flu during autumn and winter.
Most chest infections are mild and clear up on their own, others can be severe and sometimes even life threatening.
The most common symptoms of a chest infection include a persistent cough, wheezing, breathlessness, a high temperature, coughing up phlegm, a rapid heart rate and chest pain or tightness.
The Queen recently returned from an official overseas tour to Australia and Samoa (Aaron Chown/PA)
People may also experience headaches, aching muscles and tiredness.
To ease symptoms people are advised to get plenty of rest and drink lots of fluids.
They can also use painkillers to relieve headaches and muscle pain as well as to bring down a fever.
Home remedies can also include sleeping with extra pillows to make breathing easier and a hot lemon and honey drink to soothe a sore throat.
Camilla, 77, is understood to be under doctors’ supervision.
The main types of chest infections are bronchitis and pneumonia.
According to NHS Inform, most bronchitis cases are caused by viruses while most pneumonia cases are due to bacteria.
If The Queen has been diagnosed with a bacterial chest infection, then she will be treated with antibiotics.
These drugs work by killing the bacteria.
Antibiotics can be prescribed with or without steroids, which help open the airways in the lungs and make breathing easier.
For viral chest infections, antibiotics are not advised. But people with viral chest infections can be prescribed steroids.
A number of groups can be at higher risk of developing serious chest infections, these include babies and very young children, children with development problems, people who are very overweight, elderly people, pregnant women, people who smoke and people with long-term health conditions.
Queen Camilla used to be a heavy smoker – a habit her husband Charles loathed – but she gave up many years ago.