MS is a chronic disease of the central nervous system, which is made up of the brain and spinal cord. MS is unpredictable and can range from benign to disabling. Some people with MS may be mildly affected. Others may lose the ability to write, speak, or walk. In MS, the immune system attacks the layer of protein called myelin that surrounds the nerve fibers in the central nervous system. Myelin insulates the electrical signals the nervous system uses to communicate. When this insulation is destroyed, communication is interrupted. Parts of the nervous system become permanently damaged.