The central nervous system in the human body contains millions of nerve cells that are joined together by nerve fibers. Tiny electrical impulses originate in these nerve cells and travel along the nerve fibers to and from the brain. These nerve fibers are coated and protected by a fatty substance know as myelin. Myelin sheaths play a role similar to insulation shields in electrical wires.

In people suffering from multiple sclerosis, the cells producing the myelin sheath are mistakenly attacked and destroyed by the immune system. This results in the myelin becoming inflamed and swollen and getting detached from the fibers. The detached myelin may eventually get destructed. Hardened (sclerosed) patches of scar tissue get formed over the nerve fibers. When nerve impulses reach such a damaged area, some of these impulses get blocked or are delayed from traveling to or from the brain. This process eventually leads to degeneration of the nerves themselves, which explains the permanent disabilities that may develop in multiple sclerosis.

Doctors and researchers do not understand what causes this autoimmune reaction.

Some researchers believe that multiple sclerosis may be caused due to genetic predispositions in some cases. In others cases, it may be caused due to micro organisms mimicking certain human cell surface molecules present on the myelin sheath, as a result of which the antibodies may in turn attack the myelin sheath. Examples of such viruses, which may be responsible in these cases include, among others, Epstine Barr Virus and the Human Herpes Virus 6. AIDS patient may contract multiple sclerosis due to immune disorders.

A hypothesis that is being considered presents the argument that since this disease is common in the temperate regions, it may signify that certain climatic conditions may be triggering off biochemical reactions which may result in demyelination and thus multiple sclerosis.