HIV, or Human Immunodeficiency Virus, is a lentivirus that attacks the immune system of humans, specifically targeting CD4 cells. It is primarily transmitted through unprotected sexual contact, sharing contaminated needles, or from mother to child during pregnancy, childbirth, or breastfeeding. Untreated HIV infection can lead to the development of AIDS (Acquired Immunodeficiency Syndrome), a condition characterized by a weakened immune system and susceptibility to opportunistic infections and certain cancers.