The specific cause of MS is not fully understood. Symptoms are caused by an abnormal inflammatory attack on the nerves of the brain or spinal cord. This inflammatory response may be triggered by genetic, environmental, and viral factors that initiate demyelination.

The primary risk factors for multiple sclerosis are:

• female;
• age 20-40;
• family history of multiple sclerosis;
• Caucasians of Anglo-Scandinavian or North Sea ancestry;
• living in a geographical area with increased incidence of the disease (above the 37th parallel).

The disease is more common in temperate climates like those in the northern United States, Canada and Europe than in tropical regions. The fact that multiple sclerosis is more prevalent in certain climates leads researchers to speculate that environmental factors play a role in the development of multiple sclerosis. Research in this area has yet to provide conclusive evidence of a direct link between the environment and multiple sclerosis, but if you move from an area with higher risk to one of lower risk, you acquire the risk of your new home if the move occurs prior to adolescence. Such data suggest that exposure to some environmental agent encountered before puberty may predispose a person to MS.
Also there have been "epidemics" of MS -- for example, a group of people living off the coast of Denmark after WWII, suggesting an environmental cause.

Researchers also believe that MS may be inherited (passed on from parents to children). First, second and third degree relatives of people with MS are at increased risk of developing the disease. Siblings of an affected person have a 2%-5% risk of developing MS. Researchers believe that there is more than one gene that makes a person more likely to get MS.
Some scientists theorize that MS develops because a person is born with a genetic predisposition to react to some environmental agent, which, upon exposure, triggers an autoimmune response.

Some studies have suggested that many viruses such as measles, herpes, and the flu viruses may be associated with MS. To date, however, this belief has not been proven. There is growing evidence suggesting that hormones, including sex hormones, can affect and be affected by the immune system. For example, both estrogen and progesterone, two important female sex hormones, may suppress some immune activity. Testosterone, the primary male hormone, may also act as an immune response suppressor. During pregnancy, estrogen and progesterone levels are very high, which may help explain why pregnant women with MS usually have less disease activity. The higher levels of testosterone in men may partially account for the fact that women with MS outnumber men with MS by 2-3 to 1.