Health Conditions

What causes it?

Acquired Immune Deficiency Syndrome, or AIDS, is caused by an infection with a specific virus - human immunodeficiency virus (HIV). AIDS is the last stage of the HIV infection. When HIV enters the body, it attacks special white blood cells that are part of your body's immune system. These cells, called CD4 cells or T cells, play an important role in fighting infections as well as in directing other cells to find and destroy disease-causing organisms. After invading a cell, HIV releases an enzyme called reverse transcriptase that helps it grow and multiply inside the CD4 cell. HIV then uses another enzyme, protease, to make pieces of virus to spread throughout the body. As HIV disables and kills more and more CD4 cells, the body's ability to fight infection weakens.

AIDS is diagnosed when CD4 cell count drops below 200 cells per cubic millimeter of blood. (Healthy adults usually have CD4 counts of 1000 or more). A diagnosis of AIDS is also given when one or more of the 26 clinical conditions that often affect people with advanced HIV disease are present. Most of these conditions are infections that generally do not affect healthy people. In individuals with AIDS, these infections are often severe and sometimes fatal because the immune system is so weakened by HIV that the body cannot fight off certain bacteria, viruses, fungi, and other microbes.