US, UK, and Sweden have the highest rates. It might be genetically tied to norse populations in some way, but no known cause.
Here’s an interesting site with some of the ongoing ideas and geographic distributions:
http://www.mult-sclerosis.org/facts.html
Sorry I misread the table - the US isn’t in the top 5 with 91.7/100,000 it falls at #11 on the table.