Among 32 resident Faroese, clinical MS began between 1943 and 1973 and comprised 3 epidemics, each one significantly later in time and lower in incidence than the preceding. This is confirmed by the present division of the cases of the epidemics according to the calendar time when the patients attained age 11. The risk of MS for Faroese of Epidemic I, (those who acquired the disease from asymptomatic British troops in the World War II occupation), was 18 per 10,000. Depending on the minimum population number required for transmission, the MS risk for Epidemic II was 15 per 18 per 10,000, and for Epidemic III (under our second model) 9 or 11 per 10,000, none differing significantly from Epidemic I. We conclude that the primary MS affection (PMSA) is a single, widespread, specific, systemic infectious disease whose acquisition in virgin populations follows 2 years of exposure starting between age 11 and 45, which then produces clinical neurologic MS (CNMS) in only a small proportion of the affected after an incubation period of 6 (virgin populace) or 12 (endemic areas) years, and which is transmissible only during part or all of this systemic PMSA phase that ends by age 27 or younger.