Antiretroviral regimens that include ritonavir-boosted protease inhibitors are commonly recommended and prescribed, particularly in patients with some degree of drug resistance. Despite the potency of boosted regimens, many HIV-infected patients receiving these regimens have incomplete viral suppression and yet maintain clinical stability and CD4 counts above nadir levels: the so called 'CD4/HIV disconnect' state. It is likely that this state of CD4/HIV discordance is due in part to the maintenance of drug resistant HIV virus that is relatively unfit, that is its replication capacity and ability to infect and destroy CD4 cells is compromised. The selective pressure exerted by antiretroviral therapy appears to be important in maintaining these drug resistant but relatively unfit quasispecies. It has been shown that even patients with CD4 counts below 50 cells/cc and ongoing viremia maintain a clinical benefit from continued therapy.