CD4 and CD8 are the cells most commonly infected by the human immunodeficiency virus, or HIV. A high CD4 or CD8 ratio would indicate that the disease is progressing slowly or that the infection is recent and the immune system is still strong.
Measurements in CD4 and CD8 counts are considered low if they fall below 200 cells per cubic millimeter, though some doctors will begin preventive measures when counts fall below 350. In general, CD4 and CD8 counts decrease as the HIV disease progresses. It is possible to have a change in these counts even though the patient's health status has not changed.