AIDS, also known as Acquired Immunodeficiency Syndrome, is primarily caused by the HIV virus. The virus is mainly found in the blood, semen, vaginal secretions, peritoneal fluid, cerebrospinal fluid, and breast milk of people infected with AIDS. It is primarily transmitted through the following three pathways: sexual contact, blood and blood products, and mother-to-child transmission. If one is infected with AIDS, it is mainly through these three routes.