HIV is a virus spread through certain body fluids that attacks the bodys immune system, specifically the CD4 cells, often called T cells. Over time, HIV can destroy so many of these cells that the body cant fight off infections and disease. These special cells help the immune system fight off infections. Untreated, HIV reduces the number of CD4 cells (T cells) in the body. This damage to the immune system makes it harder and harder for the body to fight off infections and some other diseases. Opportunistic infections or cancers take advantage of a very weak immune system and signal that the person has AIDS.