HIV (human immunodeficiency virus) is a virus that attacks the body's immune system, specifically CD4 cells, which play a crucial role in fighting infections. HIV can be transmitted through unprotected sex, sharing of needles, or from mother to child during pregnancy, childbirth, or breastfeeding. If left untreated, HIV can progress to AIDS (acquired immunodeficiency syndrome) and lead to severe damage to the immune system, making the body vulnerable to various infections and illnesses.