Normally, nerves are surrounded by an outer layer called myelin. Myelin protects the nerve and allows it to conduct impulses faster than it would otherwise, much like the plastic insulation that surrounds a wire.

In Multiple Sclerosis areas of myelin are damaged or destroyed, a process called "demyelinization". This may occur in the brain or the spinal cord. Nerves outside of the brain and spinal cord, the peripheral nerves, are not involved. The symptoms caused by MS depend on what areas of the brain or spinal cord are involved, and vary from person to person.

Multiple Sclerosis is felt to be caused by an abnormal immune system. In effect, the body somehow becomes "allergic" to itself and uses the immune system to destroy the tissue it considers foreign. This type of condition is called "auto-immune." There is also inflammation of the nerves and this contributes to the ongoing problems.

An "exacerbation" is a period of time when the symptoms get worse. This corresponds to increased destruction of the Myelin and possibly nerves as well.

A "remission" occurs when the symptoms get better and may actually go away. This occurs because the immune system is no longer attacking the myelin and the may be able to repair itself.

Most MS patient experience exacerbations and remissions of their disease.