Human immunodeficiency virus HIV attacks the immune system, leaving its host much more vulnerable to infections and diseases. If the virus is left untreated, the susceptibility to infection worsens. HIV can be found in semen, blood, breast milk, and vaginal and rectal fluids. HIV can be transmitted through blood-to-blood contact, sexual contact, breast-feeding, childbirth, the sharing of equipment to inject drugs, such as needles and syringes, and, in rare instances, blood transfusions. With treatment, the amount of the virus present within the body can be reduced to an undetectable level. This means the amount of HIV virus within the blood is at such low levels that it cannot be detected in blood tests.