The CD4 count is like a snapshot of how well your immune system is functioning. CD4 cells (also known as CD4+ T cells) are white blood cells that fight infection. The more you have, the better. These are the cells that the HIV virus kills. As HIV infection progresses, the number of these cells declines. When the CD4 count drops below 200, a person is diagnosed with AIDS. A normal range for CD4 cells is about 500-1,500. Usually, the CD4 cell count increases when the HIV virus is controlled with effective HIV treatment.