If this is true I’m guessing it will be down to him being exposed to a heavy viral load after he rather unwisely visited Cornavirus victims earlier in the outbreak in hospital and shook all their hands oblvious to the potential consequences. If he had a mild case he should have gotten over it by now.
Last I heard it may have been the EU’s Brexit point guy who bugged Boris.
Day 10 is critical in progression of the disease
That is when things can turn, fast