Human Immunodeficiency Virus (HIV) is the virus that causes AIDS. HIV makes copies of itself by slowly killing a type of immune cell, the CD4 cell. Over time, this weakens the immune system.
As a result, a person may develop a serious disease called "opportunistic infections." These occur as a sign of later-stage HIV disease called Acquired Immune Deficiency Syndrome (AIDS). A person with HIV can also be diagnosed with AIDS when the number of their CD4 cells falls to 200 or below. HIV treatments can delay the onset of AIDS for many years.