What is multiple sclerosis? Selma Blair opened up about the disease after leaving "Dancing with the Stars"

Selma Blair revealed this week she was leaving "Dancing with the Stars" due to complications from multiple sclerosis. The disease, commonly referred to as MS, affects nearly 1 million adults in the U.S. and it could be disabling. 

Most people receive an MS diagnosis between the ages of 20 and 40 and the disease is more common in women than men, according to the Cleveland Clinic, a nonprofit academic medical center.

MS is a disease of the brain and spinal cord and it can disable people because it affects the central nervous system, according to Mayo Clinic, another nonprofit academic medical center. The myelin, or protective sheath that covers nerve fibers, is attacked by the immune system, resulting in communication problems between the brain and the rest of the body.