In Britain, the Queen has contracted Covid, Buckingham Palace has announced.
The monarch, 95, has tested positive for the virus and is experiencing "mild cold-like symptoms" but expects to carry out "light duties" this week.
The head of state has fallen ill after it was confirmed she had been in direct contact with her eldest son and heir, the Prince of Wales, the week he had the disease.
The shock announcement was made just a few weeks after the nation's longest-reigning monarch reached her historic Platinum Jubilee of 70 years on the throne on February 6.
Buckingham Palace said in a statement: "Buckingham Palace confirm that The Queen has today tested positive for Covid.
"Her Majesty is experiencing mild cold-like symptoms but expects to continue light duties at Windsor over the coming week.
"She will continue to receive medical attention and will follow all the appropriate guidelines.
Concern across the nation will be at peak levels for the Queen given her advanced age and her health scare in recent months, and her popularity in a country where many have known no other monarch on the throne.
The Royal Household has its own royal physicians and the Queen's doctors will be on hand to take care of and monitor the head of state, with Professor Sir Huw Thomas, head of the Medical Household and Physician to the Queen, expected to be in charge.
The monarch carried out her first major public engagement for more than three months on Saturday February 5, the eve of her Jubilee, when she met charity workers at Sandringham House, cut a celebratory cake and used a walking stick to rest on.
The Queen is understood to be triple vaccinated but she had been on doctors' order to rest since mid October, after cancelling a run of engagements and spending a night in hospital undergoing preliminary tests.
She is believed to have spent time with Charles on Tuesday February 8, when he hosted an investiture at her Windsor Castle home, and a few days later he tested positive for Covid but made a quick recovery to full health.
The Duchess of Cornwall has also tested positive for Covid, with Clarence House confirming on Monday February 14 that the duchess was self isolating.