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It was reported months ago that he needs an operation that requires a general anaesthetic and potentially wouldn't leave him able to return to work but he agreed to postpone it until Brexit was complete, I assume this is what is being referred to
Edit - found a link
https://inews.co.uk/news/politics/dominic-cummings-could-quit-boris-johnsons-government-if-hes-unable-to-cut-down-hours-1345990
I think he's a twat, but I'm not making an exception because I have it on good authority from a friend who has edited a flagship UK news programme for a long time that he is already very ill anyway. So this won't be good news for him.