Why is the severity of hiv measured by t-cell count?

Stage of infection: Hiv attaches to proteins on the outside of t cells to gain entry; once inside, it takes over the inner workings and starts making more viruses. T cells are immune cells that help protect against a variety of infections. As HIV infects more & more t cells, their numbers decrease & opportunistic infections increase. The stage of HIV infection can be determined by the t cell count & viral load.

HIV: As CD4 t-cells decrease in numbers there is a much increased risk of opportunistic infections and the resulting complication known as aids.