Multiple sclerosis (MS) is a chronic neurological disease of the central nervous system, specifically involving the brain, spinal cord, and optic nerves. Multiple sclerosis causes problems with muscle control and strength, vision, balance, sensation, and mental functions.

The brain, spinal cord, and optic nerves are connected to one another by nerve fibers. A protein coating called myelin surrounds and protects the nerve fibers. Myelin can become inflamed or damaged; this is called demyelination. In MS, immune cells from blood vessels enter the brain, spinal cord, or optic nerves and cause areas of inflammation, demyelination, and nerve damage.

The cause of MS is not known. Although the disease has no cure, medications can reduce the frequency and severity of attacks in many people. Most people live with the disease for decades; however, many people face growing disability as the years go by.