The association between viral disease and nutrition has long been thought to be due to effects on the host immune system. This theory suggests that when a host is malnourished, the immune system is compromised, and thus increased susceptibility to viral infection will occur. However, the virus itself may also be affected by the nutritional status of the host. We have demonstrated that a normally-benign strain of coxsackievirus B3 (CVB3/0) becomes virulent in either Se-deficient or vitamin E-deficient mice. Although the deficient animals are immunosuppressed, the virus itself is also altered. Six nucleotide changes were found in the virus that replicated in the deficient mice, and once these mutations occurred, even mice with normal nutrition became susceptible to disease. Thus, the nutritional status of the host was able to transform an avirulent virus into a virulent one due to genomic changes in the virus. We believe that a common mechanism of oxidative stress is the underlying cause of the genetic changes. Both vitamin E and Se act as antioxidants, and benign virus inoculated into GSH peroxidase (EC 188.8.131.52)-knockout mice will also convert to virulence due to genomic changes. Our work points to the importance of host nutrition during a viral disease, not only from the perspective of the host, but from the perspective of the viral pathogen as well.