Prince William apparently contracted the coronavirus in April, it was revealed in news reports late Sunday.
His father, Prince Charles and the UK Prime Minister Boris Johnson also contracted the virus.
William decided to keep the infection private so as not to alarm the public.
The Sun reported William contracted the virus in early April - after Prince Charles announced he had mild symptoms.
However, the Duke of Cambridge apparently "struggled to breathe" and at one stage his symptoms knocked him hard and his royal staff and family were "pretty panicked."
The 38-year-old second in line to the throne was in the care of palace doctors while he was isolated at Anmer Hall, his country home in Norfolk. A source added that William proceeded with "business as usual" after a week-long break and was up and about to be at the virtual opening of the makeshift coronavirus hospital in Birmingham on April 16.
By the third week of April, William was on TV for Comic Relief. He did a short virtual sketch with actor Stephen Fry while wife Kate Middleton joined him for Clap for Carers.
William also accompanied Prince George and Princess Charlotte in April to distribute food parcels to isolated families and pensioners in Sandringham.
Kensington Palace reportedly refused to make an official comment.
In late March, Charles, 71, said in a news statement he was positive for the virus after arriving in Scotland to isolate with Camilla, the Duchess of Cornwall. He said he was lucky to have just mild symptoms and spent seven days in self-isolation at Birkhall in the Highlands.
On April 2, Downing Street confirmed that the prime minister's fever had risen. Johnson was in intensive care unit a few days after contracting the virus and developing a persistent cough.
Meanwhile, William's secret battle with coronavirus sparked angry reactions from some royal observers. Royal correspondent Richard Palmer posted on Twitter that it would make trusting the future king or his advisers hard after they concealed his condition.
Palmer said people won't praise leaders like Johnson, Trump or Biden for hiding their COVID-19 diagnoses to appease public worries. He said William is an "important taxpayer-funded person," implying that he owed the public some transparency.
Defenders of William, however, said he didn't lie about his condition and was responsible about not infecting others by staying isolated. They said that the country would have been informed if his condition turned for the worse.
