Multiple sclerosis (MS) is a chronic, disabling disease of the central nervous system. The disease causes inflammation, destruction, and scarring of the nerve sheath (called myelin) that covers nerve fibers in the brain and spinal cord. There can also be damage to the underlying nerve itself (called the axon). As a result, electrical signals are slowed or blocked.

Nerve cells (neurons) showing normal and damaged myelin sheaths Copyright © Nucleus Medical Media, Inc.

There are several types of MS:

• Relapsing-remitting MS—Symptoms suddenly reappear every few months or years, last for a few weeks or months, and then go back into remission. Symptoms sometimes worsen with each occurrence.
• Primary progressive MS—Symptoms gradually worsen after they first appear. Relapses and remissions usually do not occur.
• Secondary progressive MS—After years of relapses and remissions, symptoms suddenly begin to progressively worsen.
• Progressive relapsing MS—Symptoms gradually worsen after they first appear. One or more relapses may also occur.

Multiple Sclerosis Copyright © Nucleus Medical Media, Inc.

A malfunction of the body's immune system seems to be the cause of MS, but exactly why that occurs is not known. Researchers are still working to understand autoimmune diseases, in which the immune system, which normally protects the body from infection and other disease, starts attacking the body. The following conditions may contribute to MS:

• Viral or other infection
• Genetic factors (heredity)
• Environmental factors

According to the National Multiple Sclerosis Society, about 400,000 people in the US have multiple sclerosis.