Several viral infections have been demonstrated to decrease the perforin expression and/or disable perforin function of CD8+ T cells, such as Epstein-Barr virus (EBV) and avian influenza (H5N1) virus, through the abnormalities in perforin structure and inhibition of perforin activation disabling the molecule from binding to target cell membrane [12,35]. The gene discussed is CD8A; the disease is viral infectious disease.