AIDS ("acquired immune deficiency syndrome") is caused by the human immunodeficiency virus (HIV). Individuals with HIV have what is referred to as a "HIV infection". When infected semen, vaginal secretions, or blood come in contact with the mucous membranes or broken skin of an uninfected person, HIV may be transferred to the uninfected person ("horizontal transfer"), causing another infection. Additionally, HIV can also be passed from infected pregnant women to their uninfected baby during pregnancy and/or delivery ("vertical transmission"), or via breastfeeding. As a result of HIV infection, a portion of these individuals will progress and go on to develop clinically significant AIDS.