Concerns have been raised as to our diagnoses, exclusions, case ascertainment, definition of epidemics, and the role of the British occupation in the occurrence of multiple sclerosis among Faroese. We believe none of these points are substantiated, but rather that there did occur three consecutive and decreasing epidemics of clinical neurologic MS (CNMS) among native resident Faroese between 1943 and 1973, with no cases before or (so far) since. We have attributed these occurrences to the introduction into the Faroe islands of what we have called the primary MS affection (PMSA) by the British troops who occupied the islands in World War II. The first Faroese population cohort of PMSA-affected, which included the epidemic I cases, transmitted PMSA to the next cohort of Faroese comprising those attaining age 11 in 1945-1956, and they included the epidemic II cases. The second cohort thereafter similarly transmitted PMSA to the third Faroese cohort with its epidemic III cases. We conclude that 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 CNMS in only a small proportion of the affected after a 6-year incubation period, and which is transmissible only during part or all of this systemic PMSA phase that ends before the usual age of CNMS onset. In endemic MS areas both the exposure and incubation periods may be twice as long, but otherwise PMSA may have there the same characteristics as inferred for the Faroes.