The analysis of related simian immunodeficiency viruses (SIV), comparing those that are pathogenic for their hosts, such SIVmac239 or SIVmac251, to the non-pathogenic SIVagm, SIVsm or SIVmnd, has shown that, despite a sustained viral replication in both pathogenic and non-pathogenic infections, only pathogenic models display an exacerbated apoptosis of CD4 T lymphocytes [27-30], beginning during primary infection [31-33]. Here, CD4 is linked to infection.