Multiple sclerosis (MS) is a nervous system disease that slows or blocks messages between the brain and body by damaging the myelin sheath. It affects the brain and spinal cord by thinning or completely destroying the myelin sheath. MS has no known single cause but is likely due to a combination of genetic and environmental factors. Symptoms can include muscle weakness, visual disturbances, loss of balance, numbness, and pain. While there is no cure, treatments aim to slow progression of the disease.