I'd like to preemptively draw a line between two different kinds of hypothesis when it comes to hygiene:

1. The immune system is not being exposed enough to wild or even infectious content, and it needs more threats to fight off.

2. ("Old Friends") The immune system is not being exposed enough to commensal or even symbiotic organisms that we co-evolved with, throwing off its calibration and tuning.

I instinctively prefer the second, the first seems a little too simple, like some some scaled-down version of "tough love" and "spare the rod[-bacteria], spoil the child."