The hygiene hypothesis is pretty well proven now as far as I know. Its the idea that early childhood exposure to various germs is important to tune the immune system correctly. It atleast partially explains the rise of autoimmunity. From the wiki:
The rise of autoimmune diseases and acute lymphoblastic leukemia in young people in the developed world was linked to the hygiene hypothesis.
I wonder how our global lifestyle affects this too. You might have had a really dirty childhood in the UK and adapted to all the dirt and toxins there, but then you take a job in Nevada or something and you don’t have childhood immunity to anything there.
I’d love a study done on autoimmune disease prominence in expats vs local populations.
The rise of autoimmune diseases and acute lymphoblastic leukemia in young people in the developed world was linked to the hygiene hypothesis.