People who live in high-latitude countries don't get much sun in winter. This may have immunological effects which could be part of the explanation for why MS is relatively common in these countries.

They also don't make much Vitamin D in winter. Low Vitamin D levels could just be a surrogate marker for lack of sunlight rather being causal in MS.

This is based on an interesting talk that I heard recently on BBC World Service. Annoyingly, I have been unable to track it down.