Although cytotoxic CD4 T cells are often observed during chronic infections such as HIV, Epstein–Barr virus (EBV), human cytomegalovirus (HCMV), and mouse CMV (MCMV) infections (35–39), they are also readily detectable following acute lymphocytic choriomeningitis virus (LCMV), influenza virus, and ectromelia virus infections (40–44). Here, CD4 is linked to infection.