AIDS - Acquired Immunodeficiency Syndrome - is caused by HIV, the Human Immunodeficiency Virus. HIV impairs your immune system, making it less resistant to diseases and infections. HIV is transmitted through exposure to bodily fluids of someone infected with HIV. This exposure most commonly occurs by having unprotected sex, by sharing needles, through blood transfusions, or by contact with open wounds. Babies born to women with HIV can also become infected. Although there is no cure or vaccine for AIDS, treatment is available.