CD4 and viral infectious disease: In SIV-infected SMs, the resistance to disease progression has been attributed to several non-mutually exclusive specific aspects of the host-pathogen interaction, including (i) the absence of chronic immune activation, (ii) the low levels of microbial translocation from the gut, and (iii) the relative preservation from direct virus infection of specific CD4+ T-cell subsets including central memory cells, stem-cell memory cells, and follicular helper cells (6, –, 15).