MS is considered to be a multifactorial disease in which both environmental and genetic components play a role in development of the disease. Many researchers believe that there is an environmental factor (or factors), like a virus or bacteria, that triggers the immune response in MS. To date, this environmental agent(s) has not been identified.
In addition to an environmental trigger for MS, researchers believe that regulation of the immune system is defective, impairing the T cell's ability to distinguish self from non-self. It is likely that this deficiency is influenced by genes. The increased risk in relatives of MS patients is good evidence that the disease is has a genetic component that makes some persons more likely to develop MS. Some evidence suggests that certain genes of T cells may be important but other, as yet unknown, genes are clearly involved. Understanding what triggers MS and why some people are more prone than others to develop MS can lead to better diagnosis, improved detection of susceptible persons and more effective treatments.