It all starts when HIV attacks T helper cells. These are the white blood cells that help defend your body's immune system. HIV invades healthy T cells to make copies of itself.
When HIV invades a T cell, it copies its own genetic code into the cell. In effect, it takes over the cell's own genes.
Once the virus has taken over the cell, it turns the cell into a virus "factory line". Making more and more copies all the time. Then your own immune cell begins to turn out new copies of the HIV virus.