The primary target of HIV is the immune system itself, which is gradually destroyed. Viral replication actively continues following initial HIV infection, and the rate of CD4 lymphocyte destruction is progressive. Clinically, HIV infection may appear "latent" for years during this period of ongoing immune system destruction. During this time, enough of the immune system remains intact to provide immune surveillance and prevent most infections. Eventually, when a significant number of CD4 lymphocytes have been destroyed and when production of new CD4 cells cannot match destruction, then failure of the immune system leads to the appearance of clinical AIDS.
Infection with HIV is sustained through continuous viral replication with reinfection of additional host cells. Both HIV in host plasma and HIV-infected host cells appear to have a short lifespan; and late in the course of AIDS the half-life of plasma HIV is only about 2 days. Thus, the persistent viremia requires continuous reinfection of new CD4 lymphocytes followed by viral replication and infected host cell turnover. This rapid turnover of HIV and CD4 lymphocytes promotes origin of new strains of HIV within the host from mutation of HIV.