CD4 and infection: Synergistically in the uninfected partner, susceptibility to infection will be increased by any host-intrinsic factors that provide portals of entry across the mucosal barrier, since this provides access to target CD4+ T cells in the underlying tissues, or that stimulate an immune response that is either ineffective at combatting or, even worse, that promotes HIV-1 replication (Galvin and Cohen, 2004).