CD4 and infection: Three main hypotheses have been proposed to explain development of HIV latency in primary CD4 T cells [51]: i) an activated, proliferating cell becomes infected and reverts back to a resting state [52–54]; ii) an activated cell becomes infected during the narrow window of time when it is returning to quiescence [33, 52]; or iii) infection is established directly in a resting CD4 T cell [17, 24, 25, 55, 56].