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The cause of multiple sclerosis is unknown. It's believed to be an autoimmune disease, in which the body's immune system attacks its own tissues. In multiple sclerosis, this process destroys myelin — the fatty substance that coats and protects nerve fibers in the brain and spinal cord.

Myelin can be compared to the insulation on electrical wires. When myelin is damaged, the messages that travel along that nerve may be slowed or blocked.

Doctors and researchers don't understand why multiple sclerosis develops in some people and not others. A combination of factors, ranging from genetics to childhood infections, may play a role.

Dec. 15, 2012