The disorder most commonly begins between 20 to 40 years old. MS is one of the major causes of disability in adults under age 65. The location of the inflammation varies from person to person and from episode to episode.
Symptoms vary because the location and extent of each attack varies.
The exact cause of the inflammation associated with MS is unknown. It has a higher incidence in northern Europe, northern United States, southern Australia, and New Zealand than in other areas of the world. There seems to be a familial tendency toward the disorder, with higher incidence in certain family groups than in the general population. An increase in the number of immune cells in the body of a person with MS indicates that there may be a type of immune response that triggers the disorder. The most frequent theories about the cause of multiple sclerosis include a virus-type organism, an abnormality of the genes responsible for control of the immune system, or a combination of both factors.