Viral or other Infectious Agents
Some data suggest that a common virus or other infectious agent may play a role in the cause of MS. Whether it is a persistent viral infection or an immune reaction caused by a temporary viral infection in the central nervous system or elsewhere in the body is not yet known. Environmental studies suggest that some factor – probably infectious – must be encountered before the age of 15 in order for MS to develop later in life. Several viruses and bacteria, including Epstein-Barr, Chlamydia, pneumonia, measles, canine distemper, and human herpes virus-6 have been or are being studied to determine if they may trigger MS, but none have been definitively proven to do so as of yet.

Environmental Factors
Epidemiologists – scientists who study disease patterns – have learned that MS occurs more frequently in geographic locations that are farther from the equator. In an effort to understand the puzzling disease patterns found in MS, scientists continue to examine geographic, demographic, and genetic variables. For example, studies have shown that people born in a geographic location with a high incidence of MS, who move to a geographic location with a lower incidence of MS before the age of 15, will acquire the lesser risk associated with their new location. Such data suggest that exposure to some environmental factor or factors prior to puberty, such as diet, exposure to industrial toxins, or content in water or soil may predispose a person to develop MS later in life.

Genetic Factors
While MS is not believed to be a hereditary disease, having a family history of MS (particularly in a parent or a sibling) does make a person more likely to develop it. In a family in which one parent has MS, the risk that their children will develop the condition is estimated to be between 2 and 5 percent.

Immunologic Reaction
MS is generally believed to be an autoimmune disease. This means that the immune system, which normally protects us from disease and infection, reacts against normally occurring antigens (proteins that stimulate an immune response) as if they were foreign. In other words, the body mistakenly attacks itself. While some component of myelin is believed to be the target of that attack, the exact antigen remains unknown. In recent years, researchers have identified the immune cells causing the attack and some of the factors that cause them to do so, as well as some of the sites (or receptors) on the attacking cells that appear to be drawn to the myelin to begin the destructive process.