CD4 and viral infectious disease: Herein, we review the characteristics of CD4 CTL across a range of human viral infections including human immunodeficiency virus type 1 (HIV-1) (9–11, 18–20), CMV (8, 10, 12, 21, 22), Epstein–Barr virus (EBV) (23–25), influenza (26, 27), viral hepatitis (28), hantavirus (29), dengue (30–33), and parvovirus B19 (34).