The Duke caught the virus in April and allegedly struggled to breathe.
William, 38, did not want to alarm the nation after both his father Prince Charles and Boris Johnson also caught the virus.
Despite being struck down by the virus, he managed to carry out 14 telephone engagements.
He was also treated by doctors and followed Government guidelines at his home, Anmer Hall, Norfolk.
A source told : “William was hit pretty hard by the virus – it really knocked him for six.
“At one stage he was struggling to breathe, so obviously everyone around him was pretty panicked.
“After seeing medics and testing positive – which was obviously quite a shock given how fit and healthy he is – William was determined it should be business as usual though.
“He was determined to fulfil his engagements.”
More to follow…