Epidemiology and etiology of multiple sclerosis.

The author believes that the Faroese saga provides major insight into what seems to him to be the essential nature of MS: There is a specific, widespread, but unidentified, infection that we call the primary multiple sclerosis affection (PMSA). PMSA is a persistent infection that is transmitted from person to person. A small proportion of persons who has PMSA will develop clinical neurologic multiple sclerosis (CNMS) years later. Prolonged exposure is needed to acquire PMSA. Acquisition follows first adequate exposure. Susceptibility to PMSA is limited to approximately age 11 to age 45 at start of exposure. CNMS is not transmissible.PMSA transmissibility is limited to a period that is less than the usual age of onset of CNMS. On the Faroe Islands, this period is approximately from age 13 to age 26. The existence of PMSA now must be inferred from the presence of CNMS.