Since the first cases of human immunodeficiency virus (HIV) were reported in 1981, HIV has become a worldwide pandemic. HIV is a retrovirus that attacks and kills the body's critical immune cells known as helper-T and CD4 cells causing a gradual deterioration of the immune system. Once a person has been infected with HIV, or becomes HIV-positive, the virus replicates for years without symptoms. The point at which immune cells reach critically low levels and symptoms occur is denoted acquired immunodeficiency syndrome (AIDS). There is no cure for AIDS, and millions of people worldwide die from the disease every year.