There are an estimated 400,000 individuals in the United States who have MS and 1 million worldwide. The prevalence of the disease in individuals of Northern European descent is 1 in 1000, whereas in Asians the prevalance is 2 per 100,000. The highest prevalence rate is 250 cases per 100,000 individuals in the Orkney Islands, north of Scotland. Hispanic, Asian, and African Americans are less likely than Caucasians to have the disease. MS is usually sporadic, and often there is no documented family history. However, some families have shown an increased frequency of the disease and thus attention has focused on the genetic versus environmental features of the disease. The prevalence of MS varies along a north-south gradient worldwide, increasing as one moves away from the equator, even in countries with relatively homogenous racial populations. This gradient of disease prevalence suggests an environmental cause. There are exceptions to this pattern, such as Sicily and Malta, two areas in close proximity, which have a 10-fold difference in prevalence rate. Additionally, there are ethnic groups that appear resistant to the disease in high-risk areas, such as the Hutterites of western Canada and Native Americans in North America, reinforcing the genetic features of the disease. Furthermore, the north-south gradient may be explained by the migration pattern of ethnic groups, as they tended to migrate to similar climates. Studies of individuals moving from high-risk to low-risk areas after puberty show that these individuals carry the risk of the first location for developing MS, suggesting that an environmental factor may trigger the disease early in life. Clusters of cases of MS have been described, but many have not withstood further scrutiny to provide evidence that epidemics of MS occur. In summary, the data indicate that the geographical distribution of MS cannot be due to a single environmental factor, nor can genetic factors alone explain the distribution.

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