CD4 and infection: However, in contrast to pathogenic infections, SIV-infected sooty mangabeys (i) have healthy CD4+ T cell levels; (ii) do not experience mucosal immune dysfunction, avoiding depletion of T helper 17 (TH17) cells, intestinal epithelial damage and microbial translocation; (iii) maintain low levels of immune activation during the chronic infection; and (iv) achieve compartmentalization of virus replication that preserves central-memory and stem-cell memory CD4+ T cells as well as follicular TH cells1,2.