Welsh Mumsnetters, please help us out with an entirely scientific enquiry into the health of the Welsh people.
DH is entirely Welsh and I'm half Welsh, but neither of us has ever lived there. On my (South) Welsh side of the family, we have a long history of:
pernicious anaemia
lupus
vitiligo
hypothyroidism
On his (North) Welsh side, they have:
lupus
primary biliary cirrhosis
childhood purpura
All of these are either definitely or putatively linked to autoimmune causes, and DH is convinced that what he calls 'Welsh in-breeding' 
is responsible for an increased prevalence of such disorders in our families. We're not from anywhere near each other within Wales so it's not shared family history.
We would be interested to hear your anecdotal evidence! DH is more convinced of his theory than I am, let the record show.