Importantly, though, humans evolved to live in environments full of random dirt microbes. We didn't really evolve to deal with repeated respiratory infections. Spillovers from wild animals would happen and kill a bunch of people from whatever group experienced them but then there'd be nobody left to infect and that strain would die out. It's only evolutionary relatively recently that our connected communities have become big enough to sustain this sort of disease. And the rate at which diseases cross over has increased rapidly as our population, contentedness, and meat diet has. Something like half the species of common cold didn't exist in 1800.

EDIT: Actually, I think learned opinion is that it might be our modern lack of intestinal parasites that could be making our immune system paranoid rather than a lack of dirt per se? Or maybe that's just the newer position with no consensus yet.