The discovery that Keshan disease likely has a dual etiology that involves both a nutritional deficiency of the essential trace mineral selenium (Se) as well as an infection with an enterovirus provided the impetus for additional studies of relationships between nutrition and viral infection. An amyocarditic strain of coxsackievirus B3, CVB3, converted to virulence when it was inoculated into Se-deficient mice. This conversion was accompanied by six nucleotidie changes in the genetic structure of the virus so that its genome closely resembled that of other known virulent CVB3 strains. Similar alterations in virulence and genomic composition of CVB3 could be observed in mice fed normal diets but genetically deprived of the antioxidant selenoenzyme glutathione peroxidase (knockout mice).