Starting off in a nice hot climate with humidity (moist hot air).
Moving to a cold climate with low humidity (dry cold air).
Body has difficulties with the long term effects of the cold.
There is a massive difference between people (independent of culture) when you work on the equator vs those when you work in cold climates.
Physiological, mental, work productivity, invention.
All these factors are driven by your biology, so it makes sense that some diseases may take hold better if you move from hot to cold (and visa versa) climates.
Seattle and Vancouver have mild winters and are wet, so if people are really getting MS there, the latitude seems like the bigger factor.
