There are a lot of conspiracies floating around about whether or not he actually has the virus. I'm inclined to believe he does have it. But that may also be because I'm just exhausted with conspiracies in general, as the vast majority of them rarely pan out as originally suggested.
Given the incubation period of 10-14 days before symptoms showed up, it means he was unmasked, on stage with the virus during the debate, likely spreading virus while speaking.
There was decent distance between everyone on stage, but if the air is stagnant enough inside, the fact that they were in close enough contact for a prolonged period of time, it does increase the chances that Biden or even Chris Wallace could come down with it as well.
But, being that this was also happening at the Cleveland Clinic, I wouldn't doubt for a second that the people there took every precaution and had the air circulating efficiently, so maybe not.
One thing is for certain, the reality TV president rarely leaves us with a dull moment.
Odds on him recovering fast enough to do one more debate? Boris Johnson was out for awhile when he got it. Had pretty severe symptoms, too. Every case is different, though.