As indicated in the Introduction section, the three phenotypes of the Blimp-1-dependent, “exhausted” CD8+ T cell generated during certain persistent viral infections, the T-bet-dependent, aberrantly differentiated CD8+ T cell occurring in the absence of CD4+ T-cell help, and the CD25high CD8+ T cell responding to a viral infection, have shared features. This evidence concerns the gene CD4 and viral infectious disease.