The highest prevalence rates for multiple sclerosis are found in Iceland, Scandinavia, the British Isles, and the countries settled by their inhabitants and their descendants, that is, the United States, Canada, Australia, and New Zealand. This suggests that the Vikings may have been instrumental in disseminating the genetic susceptibility to the disease in those areas as well as in other parts of the world. The Vikings raided in most European countries and settled in Normandy and in Sicily and southern Italy. They engaged in trade with the Arabs along the river routes to the Caucasus and to the Black and Caspian Seas and penetrated into Persia, India, and probably China. They also migrated to the East and established the Russian state. Under the name Varangians, they became part of the Byzantine army and were active in all of the military activities of the Byzantine Empire. They participated in the Crusades. Russians, many of Scandinavian origin, also constituted a regiment of the Mongol army and roamed throughout that empire as well. The custom of capturing and keeping or selling women and children, which was widespread in the early Middle Ages, as well as the flourishing slave trade in men, were important factors in this genetic dissemination.