<P> Two types of HIV have been characterized: HIV - 1 and HIV - 2 . HIV - 1 is the virus that was originally discovered (and initially referred to also as LAV or HTLV - III). It is more virulent, more infective, and is the cause of the majority of HIV infections globally . The lower infectivity of HIV - 2 as compared with HIV - 1 implies that fewer people exposed to HIV - 2 will be infected per exposure . Because of its relatively poor capacity for transmission, HIV - 2 is largely confined to West Africa . </P> <P> After the virus enters the body there is a period of rapid viral replication, leading to an abundance of virus in the peripheral blood . During primary infection, the level of HIV may reach several million virus particles per milliliter of blood . This response is accompanied by a marked drop in the number of circulating CD4 T cells . The acute viremia is almost invariably associated with activation of CD8 T cells, which kill HIV - infected cells, and subsequently with antibody production, or seroconversion . The CD8 T cell response is thought to be important in controlling virus levels, which peak and then decline, as the CD4 T cell counts recover . A good CD8 T cell response has been linked to slower disease progression and a better prognosis, though it does not eliminate the virus . </P> <P> Ultimately, HIV causes AIDS by depleting CD4 T cells . This weakens the immune system and allows opportunistic infections . T cells are essential to the immune response and without them, the body cannot fight infections or kill cancerous cells . The mechanism of CD4 T cell depletion differs in the acute and chronic phases . During the acute phase, HIV - induced cell lysis and killing of infected cells by cytotoxic T cells accounts for CD4 T cell depletion, although apoptosis may also be a factor . During the chronic phase, the consequences of generalized immune activation coupled with the gradual loss of the ability of the immune system to generate new T cells appear to account for the slow decline in CD4 T cell numbers . </P> <P> Although the symptoms of immune deficiency characteristic of AIDS do not appear for years after a person is infected, the bulk of CD4 T cell loss occurs during the first weeks of infection, especially in the intestinal mucosa, which harbors the majority of the lymphocytes found in the body . The reason for the preferential loss of mucosal CD4 T cells is that the majority of mucosal CD4 T cells express the CCR5 protein which HIV uses as a co-receptor to gain access to the cells, whereas only a small fraction of CD4 T cells in the bloodstream do so . A specific genetic change that alters the CCR5 protein when present in both chromosomes very effectively prevents HIV - 1 infection . </P>

The virus of aids affects the growth of