I don't know man, it's been a long time since 8th grade biology but I think it has something to do with viruses, these little bits of RNA, not even DNA, that get into our bodies and fuck with us.
I mean if he's sick with anything it's smart not to have him around anybody. he still has a day to get fluids and feel better. If it's COVID he's fucked, in any other scenario I can't see him missing this game. This is baseball not the Tour De France if he's got flu like symptoms he should be able to focus for a few ABs especially if he has to DH.
They are doing it for the stadium experience as well. I personally think it's a good idea not to try and play a bunch of games in October late at night in the NE and Midwest. Nobody really wants a 48 degree drizzle in the dark.