Multiple sclerosis is a chronic inflammatory demyelinating condition of the nervous system. Its etiology is obscure. Something in the environment strikes a susceptible host. Somehow the immune system produces central nervous system lesions. Magnetic resonance imaging scans show far greater "lesion" formation in the brain than was expected from clinical exacerbations. White blood cell changes suggest the disease is not entirely localized to the CNS. Some immunomodulatory treatments temporarily slow the rate of deterioration.