A scientist's hands holding three test tubes. The one on the left contains normal healthy t-lymphocytes, notice the pellet on the bottom. The middle vial has t-cells that have been infected with the AIDS virus HIV also known as HTLV-III; note the pellet has been destroyed since the cells have died. The test tube on the right contains t-lymphocytes that were exposed to the AIDS virus but since they were protected by AZT, the cells were not destroyed and the pellet stayed intact. Photo by John Cra