What infectious agent causes AIDS?

AIDS (acquired immunodeficiency syndrome) is caused by infection with the human immunodeficiency virus (HIV), a retrovirus that specifically targets and progressively destroys CD4+ T lymphocytes—key immune cells essential for coordinating adaptive immune responses. HIV enters host cells via binding of its envelope glycoprotein gp120 to the CD4 receptor and a coreceptor (typically CCR5 or CXCR4) on the surface of susceptible cells, including helper T cells, macrophages, and dendritic cells. Once inside, the virus reverse-transcribes its RNA genome into DNA, integrates it into the host genome using viral integrase, and hijacks cellular machinery to produce new virions. Over time, chronic viral replication leads to profound CD4+ T-cell depletion, resulting in severe immunosuppression. This compromises the body’s ability to mount effective immune responses against opportunistic infections (e.g., Pneumocystis jirovecii pneumonia, cytomegalovirus retinitis) and malignancies (e.g., Kaposi sarcoma, primary central nervous system lymphoma), defining the clinical stage of AIDS.