People who are exposed to certain forever chemicals may be at greater risk of developing multiple sclerosis (MS), according to new research.

No one knows why that is, but it could help explain why, over the past 30 years, the prevalence of MS has increased by an average of 26 percent globally. In some nations, cases have more than doubled since 1990.

MS is an autoimmune disease of the central nervous system with no known singular cause and no known cure.