CD4 count <200/mm3 when starting cARTOlder ageCo-infection (e.g. TB, hepatitis C virus, HIV-2, human T-cell leukemia virus type 1 [HTLV-1], HTLV-2)Medications, both ARVs (e.g. ZDV TDF+didanosine [ddI]) and other medicationsPersistent immune activationLoss of regenerative potential of the immune systemOther medical conditions. The gene discussed is CD4; the disease is infection.