Further analysis of CD4 and CD8 counts and ratios in smoke-exposed HIV-infected groups (smoke vs. HIV + smoke) and smoke combined with morphine exposure group (smoke + morphine vs. HIV + smoke + morphine) at 8-week time-point, revealed much greater reduction of CD4 counts and CD4/CD8 ratios, indicating that HIV infection in the background of smoke and morphine exposure worsened the CD4 T cell decline (Fig. 2C). Here, CD4 is linked to HIV infectious disease.