Multiple sclerosis (MS) is thought to be an immune-mediated disease with a possible environmental trigger. Genetic and environmental factors, including infection by pathogens, may act synergistically to trigger the disease. There is growing epidemiologic, serologic, and pathologic evidence that Epstein-Barr virus may cause MS or contribute to its pathogenesis. The evidence that canine distemper virus is involved in MS is less robust. More definitive data are required to prove that Epstein-Barr virus or canine distemper virus causes some or most cases of MS.