The cause of multiple sclerosis (MS) is unknown. However genetic factors play at least some role. An individual's risk of developing MS increases several-fold if a close family member has MS. While the average person in the United States has about 1 chance in 750 of developing MS, the risk for a person who has a parent with MS increases to 1 in 40. MS. Thus, the risk increases significantly for a person whose parent has MS, but still remains relatively low.

There is a relationship between a where a person grows up during childhood and the risk of MS later in life. MS seems to be more common in northern latitudes. The incidence of MS is 30-80 times higher in northern Europe and northern North America than near the equator. In addition, there is some evidence suggesting that people who migrate from a high-risk area to a low-risk area before the age of 15 take on the risk associated with their new area. If they are older than 15, they retain the risk associated with their old area. These observations suggest that there may be infectious or environmental factors at work in the disease. While many infectious agents (especially viruses) have been suspected as causative in MS, none have ever been proven and the theories currently most accepted are that a problem with the body's natural defense system (immune system) occurring later in life may trigger the onset of MS in people who were exposed to certain factors during childhood. This causes an autoimmune reaction in which the immune system attacks myelin (the protein coating that protects nerve fibers). Other factors, such as diet, have been proposed as causative of MS but most evidence supports this autoimmune cause.

see also causes MS.asp NMSS: What Causes MS