CD4 and viral infectious disease: The circulating CD4+ cytotoxic T lymphocytes (CD4-CTLs) have been better studied in viral infections in both humans and animal models, which include human cytomegalovirus, Epstein-Barr virus, human immunodeficiency virus (HIV), dengue virus, influenza virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV2), and many more [reviewed in (12)].