In untreated pts or in pts in whom therapy has not controlled viral replication (see below), after some period of time (often years), CD4+ T cell counts will fall below a critical level (~200/µL) and pts become highly susceptible to opportunistic diseases. The presence of a CD4+ T cell count of <200/µL or an AIDS-defining opportunistic disease establishes a diagnosis of AIDS. Control of plasma viremia by effective antiretroviral therapy, particularly maintaining the plasma viral load consistently at <50 copies of RNA per mL, even in individuals with low CD4+ T cell counts, has dramatically increased survival in these pts, including those whose CD4+ T cell counts may not increase significantly as a result of therapy.