Final answer:
The best indicator of the effect of an intervention in the treatment of HIV infection is the CD4+ T-cell count.
Step-by-step explanation:
The best indicator of the effect of an intervention in the treatment of HIV infection is the CD4+ T-cell count. CD4+ T cells are a type of white blood cell that play a crucial role in the immune response against HIV. Monitoring the CD4+ T-cell count allows physicians to assess the patient's immune function and adjust treatment accordingly.