alt Hacker NewsI'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."
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An excellent distinction to make. Life however often says "Why not both? And 11 more you'd have never thought of. And one that seems impossible just for fun."
If it's possible, and it can force a function up a gradient, life is almost certainly doing it somewhere.
Regarding the second point: ...or parasitic.
There's a hypothesis that says the incidence of allergies correlates inversely with the incidence of certain common parasites, like the tapeworm or the pinworm. Additionally, nowadays pregnant women are advised to avoid getting infected with toxoplasmosis due to the birth defects it causes, but it wasn't until the 70s when the last route of transmission was found and explained.
What if the body is just looking for parasites where there are none?
EDIT: I also lean on the second, as the first doesn't explain why allergies can come and go seemingly without reason.
Personally currently I'm allergic to some unindentified plant and it's a different one than back when I was a child. Meanwhile my child is right now experiencing "my" childhood allergy season - with similar severity at that.