CD4 and infection: Other factors that contribute to this diversity include: host immune response (APOBEC enzyme hypermutations) (Bruner et al., 2016), length of infection (accumulation of mutations and depletion of CD4 T cell targets) (Arrildt et al., 2015; Zhou et al., 2016), viral compartmentalization (Yukl and Wong, 2015; van Zyl et al., 2018; Miller et al., 2019), and selection for viral quasispecies that are able to evade killing by host immune mechanisms (Phillips et al., 1991; Wei et al., 2003; Kearney et al., 2009; van Zyl et al., 2018).