CD4 and infection: SIVmac239 infection induced a more dramatic decrease in the CD4+ T cell count (265.6 CD4+ T cells/μl) and CD4/CD8 ratio (0.36) in old ChRM compared with young macaques (468.1 cells/μl and 0.74; P = 0.0027 and 0.02, respectively) (Fig. 2a).