HIV is the abbreviation used for the human immunodeficiency virus. HIV is the virus that causes AIDS (acquired immunodeficiency syndrome), a life-threatening disease.
HIV attacks the body's immune system. The infection-fighting cells of the immune system are called CD4 cells or T-helper cells. Months to years after a person is infected with HIV, the virus destroys the CD4 cells. When the CD4 cells are destroyed, the immune system can no longer defend the body against infections and cancers.

HIV infection becomes AIDS when you lose your ability to fight off serious infections or tumors. Various infections called opportunistic infections develop. They are called opportunistic because they take advantage of the weakened immune system. These infections would not normally cause severe or fatal health problems. However, when you have AIDS, the infections and tumors are serious and can be fatal.