I think there is a big difference between exercising your immune system and creating a Petri dish for air borne diseases. If COVID has taught us anything, it’s that ventilation in many buildings sucks big time. We need to either improve ventilation, or start killing viruses.

I mean, if you’re going to go the naturalistic angle, then what’s natural about squashing people into buildings with poor air flow?

Or maybe the 20th century increase in allergies was caused by the sheer amount of bad stuff we created, or maybe it's because of the sheer concentration of badly ventilated environments where hundreds of people pass through every day.

AFAIK, there is no good evidence about any of those hypothesis, and weak ones for them all.

let me know how your exercise with avian flu, yellow fever, HIV, ebola, hepatitis, etc etc etc goes.