Without much thought regarding cost. Calm down lol

What evidence is there that it’s possible to “exercise” an immune system?

What recent experience? Do you mean keeping them home from school for covid? I don't think the knock on effects from that are remotely traceable to their immune systems.

As mentioned elsewhere in the thread, one can be exposed (and have an immune response) to a viral load below what's normally considered symptomatic. Then the immune system learns, but doesn't have to go into over-drive which taxes the rest of the body more destructively.

In other words: Filtering part of it from the air can make exposure more like vaccination than full blown infection, which is great.

Getting sick to stay healthy. Makes sense to me. Now will you be exposing yourself to zika virus or HIV? Seems like those would exercise your immune system quite a bit.