[Volundr]

I'm not sure why you immediately assume this is hypothetical. I know a couple people in similar situations. They exist and are not so uncommon as to assume the OP is making it up.

It's also not the doom scenario you describe. Yes they have to take extra precautions, and yes the people around then have to make a point to not be around when they are sick, but with a little consideration plenty of people with highly compromised immune systems live meaningful lives.