HIV is primarily spread by '''sexual contact ''' and '''intravenous drug use'''. Most early infections in the US were via homosexual sex; most current infections in the world are via heterosexual contact and '''vertical transmission from mother to child'''. Mothers infected with HIV transmit the virus to their baby ''in utero'', during childbirth, or via breastfeeding. Mother-to-child transmission can be significantly reduced by the proper use of antiretroviral agents.
Less commonly, contact with infected blood causes HIV transmission. This can occur in health care providers (HCPs) or others exposed to infectious bodily fluids. Transmission is facilitated by breaks in the skin or direct contact with mucosal tissues, such as those found in the eyes, mouth, anus, or vagina. Early in the epidemic, blood transfusions were a significant source of HIV transmission.