A neuromuscular disorder that characteristically involves the destruction of myelin, the insulating material around nerve fibers. The onset of the disease is unusual in persons under 15 or over 60 years of age, and peak incidence is found in people in their 20s and 30s. Multiple sclerosis affects females more frequently than males by approximately 2:1. Distribution is worldwide, but there is an unusual relationship to latitude, with a much higher incidence at northern latitudes than near the Equator.