One of the two types of HIV, the virus that causes AIDS. AIDS is the most advanced stage of HIV infection. HIV-1 is transmitted through direct contact with HIV-infected body fluids, such as blood, semen, and genital secretions, or from an HIV-infected mother to her child during pregnancy, delivery, or breastfeeding (through breast milk). HIV-1 is responsible for the majority of HIV infections worldwide. In the United States, unless otherwise noted, the term “HIV” primarily refers to HIV-1.