Final answer:
CD4 T-cell levels may begin to decline after initial treatment with AZT due to the HIV virus developing resistance to the medication and its capability to hide in latent reservoirs within immune cells.
Step-by-step explanation:
After 2 years of using AZT, a type of antiretroviral medication, CD4 T-cell levels of patients began to fall again. This decline can occur because HIV, the virus responsible for AIDS, is capable of becoming resistant to medications over time, including AZT. The virus continues to replicate, albeit at reduced levels, and as it mutates, it can develop resistance to the drugs being used to suppress it. Consequently, the viral load increases, and CD4 T cell levels drop, weakening the immune system and making the patient more susceptible to opportunistic infections. Additionally, HIV has a reservoir of latent virus within the body's immune cells, which can begin to replicate if drug treatment is interrupted or becomes ineffective.