The Queen has revealed that her chest infection was a form of pneumonia.
Camilla, 77, confirmed the diagnosis to guests as she attended a lunch at Buckingham Palace with the King, and the Prince and Princess of Wales, as the royals hosted the Emir of Qatar for a state visit.
It comes after the Queen's schedule was adjusted slightly as she is suffering from post-viral fatigue.
She missed the outdoor elements of the Qatari visit on Tuesday, including the ceremonial welcome at Horse Guards Parade, after being urged by her doctors to take more time to recover.
Instead, Camilla, dressed in a mauve frock, was driven to the palace by car and waited for the Emir and his first wife inside the entrance of the Grand Hall.
It is understood the Queen was not taken to hospital with pneumonia, but is now facing bouts of extreme tiredness.
© GettyA royal source said: "Some days the Queen is on really good form. It's just one of those things. She has lost the coughing but the lingering side of it is bouts of extreme tiredness. There has to be a degree of flex in the Queen's diary at the moment."
Camilla contracted her chest infection after a long-haul trip to Australia and Samoa in October and has missed a number of engagements since then, including the Royal Variety Performance, the Gladiator II premiere and the Remembrance Day service at the Cenotaph.
It is understood the Queen's condition was viral, not bacterial, and she no longer has pneumonia.
© GettyMeanwhile, the Princess of Wales made another major public appearance amid her gradual return to her royal duties.
After attending Remembrance events last month, Kate joined Prince William to greet Sheikh Tamim bin Hamad Al Thani and Sheikha Jawaher from their private residence to Horse Guards Parade for the ceremonial welcome.
© GettyThe Princess, 42, looked beautiful in a burgundy coat dress by Sarah Burton for Alexander McQueen and a matching hat by Sahar Millinery. She accessorised with the late Queen's four-strand pearl choker necklace and the Bahrain pearl drop earrings, finishing her look with a Chanel bag.
What is pneumonia?
Pneumonia is inflammation of the lungs which is usually caused by an infection such as flu, the common cold or other viruses.
Older people, babies and people with heart or lung conditions are at risk of becoming seriously ill due to the condition.
Symptoms include a cough, shortness of breath, fever, chest pain, aches, fatigue, a loss of appetite, wheezing and feeling confused.
Many people diagnosed with pneumonia will be given antibiotics to treat the condition.
Some people with chest infections can be prescribed steroids, which help open the airways in the lungs and make breathing easier.
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.
