Multiple sclerosis is thought to be due to an autoimmune reaction to an exogenous stimulus such as a virus. Oligodendroglia are not primarily affected and may even multiply to remyelinate the periphery of the lesion the first time. However, its prevalance varies with latitude being higher the farther from the Equator one lives. It is related to exposure to the agent before 15 years of age. People who migrate from a high to low before 15 take on the low prevalance whereas, those that migrate from a high to low area after 15 maintain the high prevalence.
Women develop the disease approximately twice as often as men.
The onset of the disease is between 20 and 40 years of age. The lesions appear as greyish plaques randomly distributed throughout the nervous system. Most commonly they are found periventricularly but can be seen anywhere including predominately grey matter. Acute MS plaques appear as areas with many macrophages filled with lipid and a few perivascular lymphocytes as well as lymphocytes in the tissue. As time passes, the macrophage reaction decrease and there is proliferation of reactive astrocytes forming fibers throughout the plaque.