According to the Public Health Agency of Canada, the name of the pathogen that leads to acquired immune deficiency syndrome (AIDS) is the human immunodeficiency virus (HIV). The two are not entirely distinct illnesses, with AIDS indicating the late stages of infection with HIV.
Once a person is infected with HIV, her immune system gradually breaks down as CD4 receptors become either destroyed or functionally impaired. Eventually, the immune system's breakdown leads to the symptoms and infections that characterise AIDS. HIV infection occurs through sexual transmission 90 percent of the time. Blood transfusions, needle sharing and receptive anal intercourse carry the highest risk per-act, with oral sex, insertive anal sex and penile-vaginal contact carrying a lower, but still significant, risk per-act.