What causes AIDS?

HIV infection, which can progress to acquired immunodeficiency syndrome (AIDS), is caused by the human immunodeficiency virus (HIV). HIV is a retrovirus that specifically targets and destroys CD4+ T lymphocytes—critical immune cells responsible for coordinating the body’s adaptive immune response. As the virus replicates and depletes these cells over time, immune function progressively deteriorates. AIDS represents the most advanced stage of untreated HIV infection, defined clinically by either a CD4+ T-cell count falling below 200 cells/μL or the development of one or more opportunistic infections or malignancies—such as Pneumocystis jirovecii pneumonia, Kaposi sarcoma, or invasive cervical cancer—that rarely occur in individuals with intact immunity.

HIV is transmitted through direct contact with certain infectious bodily fluids—including blood, semen, vaginal and rectal fluids, and breast milk—from a person living with HIV who has a detectable viral load. Common transmission routes include unprotected sexual intercourse, sharing contaminated needles or syringes, mother-to-child transmission during pregnancy, childbirth, or breastfeeding, and, less commonly today due to rigorous screening, transfusion of unscreened blood products. Importantly, HIV is not transmitted through casual contact such as hugging, shaking hands, sharing food, or via air, water, or insect bites.

Early diagnosis and lifelong antiretroviral therapy (ART) can effectively suppress viral replication, preserve immune function, prevent progression to AIDS, and reduce transmission risk to near zero when viral suppression is sustained. With timely and consistent treatment, people living with HIV now have life expectancies approaching those of the general population.