Multiple sclerosis (MS) is a disease of the central nervous system in which there are multiple areas of inflammatory demyelination with a predilection for distribution around the ventricles and vascular spaces of the brain. Lesions representing areas of inflammatory demyelination can be present in the cerebral hemispheres, brainstem, and spinal cord.

MS is a very common disease with a prevalence ranging from 20,000-25,000 people. Studies of migrating populations have indicated that where one resides in relation to the equator before the age of 15 appears to determine the likelihood of developing MS. The incidence of MS is nearly 3/100,000 in temperate zones, and below 1/100,000 in tropical areas, with women more affected than men in a ratio of approximately 2:1. Persons who have a definite diagnosis of MS are more likely to have a variety of other illnesses of an autoimmune nature such as systemic lupus erythematosus, rheumatoid arthritis, polymyositis, myasthenia gravis. Furthermore, if a first-degree relative has MS, there is a 12-20-fold increase in the likelihood of having MS. In monozygotic (identical) twins there is a 33% increase in the incidence of MS, whereas in dizygotic (fraternal) twins the incidence is about 8% greater than the general population.

For a definite diagnosis to be established, two or more areas of demyelination (white matter lesions) must exist. Furthermore, there must be two or more remissions of neurological deficits. This must be accompanied by laboratory evidence of disease seen using magnetic resonance imaging (MRI), as well as evidence of increased IgG synthesis with positive oligoclonal bands (OCBs) in the spinal fluid.