Human immunodeficiency virus (HIV) is the cause of AIDS (acquired immunodeficiency syndrome). HIV screening tests detect the HIV antigen (p24) and/or HIV antibodies produced in response to an HIV infection in the blood. Some tests detect HIV antibody in oral fluid. If HIV is not detected early and treated, it may become a simmering infection that may cause few symptoms for a decade or more. If the infection is still not treated, eventually symptoms of AIDS emerge and begin to progressively worsen.