CD4 and infection: Remarkable research efforts have led to a better understanding of HIV target cells (mainly CD4+ T cells), and HIV cellular reservoirs, including naïve CD4+ T cells (TN), stem cell-like memory (TSCM), central memory (TCM), transitional memory (TTM), effector memory CD4+ T cells (TEM), T helper 1, 2, 17, 9 (Th1, Th2, Th17, Th9) cells, regulatory T cells (Treg), follicular T helper cells (Tfh), astrocytes, dendritic cells (DCs), and tissue-resident macrophages which establish in different tissues soon after acquiring infection [21,22].