CD4 and HIV-1 infection: Furthermore, we demonstrate that although the initial infection is dependent on target cell frequency, once infection has occurred, it recapitulates the salient features of the paradigm of HIV-1 infection that have been proposed to occur following heterosexual infection in women, including local viral amplification followed by systemic spread, decline in vaginal titres, increase in plasma titres and decline in circulating CD4+ target cells over time, regardless of reconstitution level.