Multiple sclerosis (MS) is a chronic disease thought to occur when the immune system attacks myelin, a protective substance that surrounds nerve fibers in the brain, spinal cord, and optic nerves, causing symptoms ranging from mild numbness in the limbs to paralysis and vision loss. About 85 percent of people are initially diagnosed with relapsing-remitting MS (RRMS), a form characterized by flares of neurologic problems followed by periods of improvement or even no symptoms (remissions. About 10 percent of newly diagnosed people have a form of the disease called primary-progressive MS (PPMS), which causes slow, steady worsening of neurologic function without any remissions. Around 5 percent are diagnosed with progressive-relapsing MS (PRMS), and experience a steady decline from the beginning, with periodic flares and no remissions. More than half of people with MS start out with RRMS, but eventually their disease becomes progressive with no remissions, which is called secondary-progressive MS (SPMS).