Major research advances have substantially improved our understanding of the biology of HIV and the pathogenesis (i.e., origin and development) of AIDS. The pathogenesis of AIDS is now known to result from the ability of HIV to replicate at the rate of a billion new virions (viral particles) per day and nearly 10 trillion new virions over the course of HIV infection. This, countered by the ability of the body to produce CD4+ T cell lymphocytes (a primary target cell for HIV), sets the stage for the struggle between HIV and the immune system-a struggle that lasts from the first day of HIV infection to end-stage disease and death.