A bit of possible support for the hygeine hypothesis...

[Guest guest]

....which is, roughly, that you need exposure to pathogens in order to

somehow hone or prime your immunity:

" A higher incidence of inflammatory bowel disease was ecologically

associated with a higher average family income [equals less exposure

to germs on average], a lower proportion of immigrant and Aboriginal

Canadian populations [whites have the highest IBD rates], and a

smaller average family size [equals less exposure to germs,

particularly germs carried by your genetic siblings, which are

well-adapted to infect you, whether symptomatically or

sub-symptomatically]. "