This thin-section transmission electron micrograph (TEM) depicted the ultrastructural details of a number of human immunodeficiency virus (HIV) virus particles, or virions. A member of the genus Lentivirus, HIV is separated into two serotypes, HIV-1 and HIV-2, and is the cause for the disease known as acquired immunodeficiency syndrome, or AIDS. This virus may be passed from one person to another when infected blood, semen, or vaginal secretions come in contact with an uninfected person's broken skin or mucous membranes*. In addition, infected pregnant women can pass HIV to their baby during pregnancy or delivery, as well as through breast-feeding. People with HIV have what is called HIV infection. Some of these people will develop AIDS as a result of their HIV infection.