CD4 and viral infectious disease: CD4+ age-associated T cells (TAAs) include effector/memory T cells; a population of regulatory T (Treg) cells expressing activation genes (denoted as activated Treg cells or aTreg cells); cells with an exhaustion signature (denoted as exhausted); and cells overexpressing genes associated with cytotoxicity that have previously been described in the context of viral infections and cancer as CD4+ cytotoxic T cells (denoted as cytotoxic)5.