MS was just terribly rare in our Grandparent's time and incidence has drastically increased since then and it's said that genetics in humans isn't capable of changing that quickly so if it really is congenital, it makes you wonder how it came to happen.
Generations of living in conditions where sunlight (and exposure to the sun) is less prevalent - and less people work outdoors regularly to get vitamin d. Vitamin D lacking then could be a cause of CCSVI (don't know yet, not that far into the research, but it's not hard to draw the link for sure) ... Vitamin d may be critical in forming proper veins in the uterus - passed from mother to child in the womb.
The sun drives life on earth ... makes things healthy, feeds them on a cellular level ... it's not a far reach to assume the sun is important in development and health. And with the known Vitamin D relation to MS - the sun obviously plays a role ... so again, it's not a far leap to connect vitamin D to (potentially) vein development and CCSVI ...
hence the 38th parallel or whatever it is
hence the high incidence of "MS" in babies born in late spring/early summer when most of the pregnancy would have been spent indoors from the cold
hence when people began getting indoor jobs, the industrial revolution, MS rate increased dramatically - in a few generations as you point out
hence when Iranian women were forced to cover head to toe, MS incident rate increased dramatically in GENERATIONS
Huh, what if CCSVI is caused by lack of vitamin d in the mother passed to the womb ... what a new neat idea that should certainly be explored.
Pittsburgh/Western PA is the 2nd cloudiest place in the US ... high incidence of MS.
Same through western NY and the great lakes regions ... very cloudy, very cold ...
Same for Canada (very cold much of the year)
Same for the pacific northwest - lots of gray days.
Same for England/Scotland - very cloudy. Little sun.
Same goes for Iranian women increasing in prevalence (when women became forced to cover up from head to toe and get VERY limited exposure to sun)
Little MS in Mexico (or other third world areas, as you point out - though I'm not sure why people can't make the connection I did above) ... lot of out door work - or whatever people in sub-saharan africa do during the day, still because, plenty of sun. Same goes for down through central america ... same goes for islanders ... little rate of incidence among african americans ... (generations haven't been not getting sun long enough though ... whereas caucasians have for many many generations)