Source Yahoo! News:
Multiple sclerosis is one of the commonest disabling neurological disorders, in which plaques of demyelination (degeneration of myelin sheaths of nerve fibres) develop in various parts of the brain and spinal cord. It is variable in its incidence and severity, in that some cases are relatively mild, some run a relapsing and remitting course with substantial intervals between episodes, which nevertheless lead to progressive disability, while in other patients the condition is rapidly progressive, with few if any remissions. Read more here.