In this study, using two human isolates of H5N1 viruses that differ in their pathogenicity in mice, we have systematically examined the following: 1) the relationship between the speed of H5N1 virus replication and viral pathogenicity on the dynamics of CD8 T cell responses, 2) whether ineffective control of H5N1 virus infection is related to the suppression of virus-specific CD8 T cell responses, 3) the effect of CD8 T cell deficiency on host survival, and 4) the effect of anti-viral therapy on CD8 T cell responses. This evidence concerns the gene CD8A and viral infectious disease.