Here it is proposed that deprivation of sunlight and vitamin D aggravates the genetically determined CD8+ T cell deficiency and impaired control of EBV infection and thereby contributes to the high prevalence of autoimmune diseases such as MS, type 1 diabetes, and RA at high latitudes [82–84] (Figure 3) and that the protective effect of vitamin D against autoimmune diseases [85–87] is mediated at least in part by an increase in the number of CD8+ T cells available to control EBV. Here, CD8A is linked to congenital T-cell immunodeficiency.