Viruses are the cause of several common STIs. A virus is an organism that invades, reproduces, and lives within a cell, thereby disrupting normal cel­lular activity. Most viruses are transmitted through direct contact with infectious blood or other body fluids. We begin our discussion with her­pes, the most common viral STI. Next, we describe genital warts caused by several varieties of viruses that have reached epidemic proportions in the U. S. population. We conclude with some information about viral hepatitis. AIDS, caused by HIV infection, is described in detail later in this chapter.


Herpes is caused by the Herpes simplex virus (HSV). Eight different herpes viruses infect humans, the most common being the varicella-zoster virus (VZV) that causes chicken pox, followed in frequency by Herpes simplex virus type 1 (HSV-1) and Herpes simplex virus type 2 (HSV-2). In the following discussion we confine our attention to HSV-1 and HSV-2 because these are the two herpes viruses that are widely transmit­ted through sexual contact. HSV-1 typically manifests itself as lesions or sores—called

cold sores or fever blisters—in the mouth or on the lips (oral herpes). HSV-2 generally causes lesions on and around the genital areas (genital herpes).

Although genital and oral herpes are usually associated with different herpes viruses, oral-genital transmission is possible. HSV-1 can affect the genital area, and, conversely, HSV-2 can produce a sore in the mouth (Centers for Disease Control, 2009f). How­ever, most infections of the genitals are of the HSV-2 variety, and most mouth infec­tions are HSV-1 (Centers for Disease Control, 2009f; Looker et al., 2008).