A CD4 T-cell is a white blood cell that plays an important part in your immune system. It detects hostile bacteria and viruses and activates other cells to clear them.
HIV uses CD4 T-cell to multiply. The HIV virus enters the CD4 cel and repeatedly makes exact copies of itself. That destroys the CD4 cell and the new HIV virus particles can infect other CD4 cells, which are then also destroyed. Once you don’t have enough CD4 cells, other viruses and bacteria are given ample opportunity to make you ill.
Antiretroviral drugs (hiv-medication) ensure that the virus in the CD4 T-cells can no longer copy itself. Then CD4 T-cells are no longer destroyed and your immune system remains intact.