Multiple sclerosis: A disease that is characterized by loss of myelin (demyelinization). Abbreviated MS. Myelin, the coating of nerve fibers, is composed of lipids (fats) and protein. It serves as insulation and permits efficient nerve fiber conduction. In MS, demyelinization usually affects white matter in the brain, but sometimes it extends into the gray matter. When myelin is damaged, nerve fiber conduction is faulty or absent, and nerve cell death may occur. Impaired bodily functions or altered sensations associated with those demyelinated nerve fibers give rise to the symptoms of MS, which range from numbness to paralysis and blindness. People with MS experience attacks of symptoms that may last days, months, or longer. For many patients, the disease is progressive and leads to disablement, although some cases enter long, perhaps even permanent, remission. The cause of MS is unknown, although viral activity is suspected. Most patients are diagnosed between the ages of 20 and 40. Rarely, children and teens develop MS (pediatric MS). Until recently, treatment had focused on preventing attacks. Steroids, interferon, and medications to treat specific symptoms (such as fatigue, depression, and vertigo) are standard, along with lifestyle changes to avoid stress and other triggers. New treatment options involve immune system modulation or support.