HIV (human immunodeficiency virus) is a retrovirus that infects cells of the human immune system (mainly CD4+ T cells and macrophages; vital components of the hosts immune system), and destroys or impairs their function. Infection with HIV results in the progressive depletion of the immune system, leading to "immune deficiency." There is general consensus that HIV is the underlying cause of AIDS.

HIV infection is acquired through penetrative (anal or vaginal) and oral sex; blood transfusion; the sharing of contaminated needles in health care settings and through drug injection; and, between mother and infant, during pregnancy, childbirth and breastfeeding.

For a full discussion of HIV and AIDS please see HIV/AIDS.