The Prince of Wales, 71, tested positive for the deadly bug while Camilla, 72, tested negative as they self-isolate in Balmoral, Scotland.
Many people wondered why the royal couple were tested for the virus, when others haven't been.
One person wrote on Twitter: "How come Charles and Camilla have been tested for Coronavirus, when he only has mild symptoms.
"Are they important or something? More important than Medics? Very puzzling."
Another said: "Please tell me how Prince Charles and Camilla got tested for coronavirus when they are not in hospital?
"I am a GP and have friends working in many hospitals.
"How come tests are not accessible for us and our colleagues?"
However, Clarence House revealed why in a statement.
Clarence House's statement on Prince Charles
The statement said the tests were carried out by the NHS in Aberdeenshire.
Charles and Camilla "met the criteria required for testing".
"In accordance with government and medical advice, the prince and the duchess are now self-isolating at home in Scotland," the statement added.
"It is not possible to ascertain from whom the prince caught the virus owing to the high number of engagements he carried out in his public role during recent weeks."
Fans expressed concerns for The Queen as she had been in contact with Charles recently.
However, Buckingham Palace insisted Her Majesty remains in "good health".
The Palace's full statement
The statement read: "Her Majesty The Queen remains in good health.
"The Queen last saw The Prince of Wales briefly after the investiture on the morning of 12th March.
"The Queen is following all the appropriate advice with regard to her welfare."
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