Definition of Multiple sclerosis:

Multiple sclerosis is an autoimmune disease that affects the brain and spinal cord (central nervous system).

Causes, incidence, and risk factors:

Multiple sclerosis (MS) affects woman more than men. The disorder most commonly begins between ages 20 and 40, but can be seen at any age.

MS is caused by damage to the myelin sheath, the protective covering that surrounds nerve cells. When this nerve covering is damaged, nerve impulses are slowed down or stopped.

MS is a progressive disease, meaning the nerve damage (neurodegeneration) gets worse over time. How quickly MS gets worse varies from person to person.

The nerve damage is caused by inflammation. Inflammation occurs when the body's own immune cells attack the nervous system. Repeated episodes of inflammation can occur along any area of the brain and spinal cord.

Researchers are not sure what triggers the inflammation. The most common theories point to a virus or genetic defect, or a combination of both.

MS is more likely to occur in northern Europe, the northern United States, southern Australia, and New Zealand than in other areas. Geographic studies indicate there may be an environmental factor involved.

People with a family history of MS and those who live in a geographical area with a higher incidence rate for MS have a higher risk of the disease.