CD4 and infection: The two-phase infection is characterized by an acute, highly destructive phase when virus massively depletes the CD4 memory T cells from effector sites in the gastrointestinal tract, followed by a chronic phase in which the damaged immune system slowly fails as constant immune hyperactivation eventually exhausts the ability for reconstitution (Li et al., 2005; Mattapallil et al., 2005; Douek et al., 2009).