HIV replicates rapidly inside the CD4 positive cells, impairing their function. In the initial stage, the rate of destruction of the cells matches the replacement. In AIDS the gap between destruction and replacement of the cells increases and immunological failure occurs. In 1983 HTLV-3 was identified as the causative agent of AIDS. It is now termed HIV-1 and is the predominant serotype worldwide. As compared with HIV-1, HIV-2 is characterized by lower rates of perinatal and sexual transmission, decreased CD4 cell killing, relative geographical confinement and slower progression to AIDS and death.