Scotland's Orkney Isles has been identified as having the highest prevalence of multiple sclerosis in the world, according to a study published in The Journal of Neurology, Neurosurgery and Psychiatry.
At a prevalence of 402 per 100,000, the likelihood of having MS is roughly 8 times higher on these islands than in the US (having a prevalence of 58 per 100,000). On these islands, 1 in 170 women have MS, with 45% having "significant disability." The islands have a small population of about 20,000 inhabitants.
No one knows why the prevalence of MS is so much higher on the Orkney Isles than in other places. It seems to be on the rise, as a study published in 1980 showed a prevalence of 309 per 100,000.