The cause of multiple sclerosis (MS) is unknown. Because a person's risk of MS is slightly higher when a relative (especially a parent, sister, or brother) has MS, there may be a genetic link.

The unusual relationship between a person's geographic location during childhood and the risk of MS later in life suggests that there may be environmental factors at work in the disease-such as viral illnesses or other infectious diseases. However, there is no clear proof that any specific infection causes MS. Other triggers for MS have been proposed, but none have yet been proven.

Furthermore, a childhood viral illness or other environmental factor by itself is probably not enough to explain why some people later develop MS. Research is beginning to suggest that a problem with the body's natural defenses (immune system) occurring early in life or after infections may trigger the onset of MS. The trigger may be an autoimmune reaction in which the immune system attacks myelin, the coating that protects nerve fibers.