serious complications. For example, bacterial vaginosis, a frequent cause of vaginitis in sexually active women, was once considered to be a benign condition but has recently been shown to be associated with premature delivery, low birth weight, and pelvic inflammatory disease (Hauth et al., 1995; Hillier et al., 1995). Human papillomaviruses, originally recognized to cause warts, are now known to be important causes of several types of cancer.
Epidemiological and other characteristics of eight common STDs are summarized in Appendix B. STDs are almost always transmitted from person to person by sexual intercourse.2 STDs are transmitted most efficiently by anal or vaginal intercourse, and generally less efficiently by oral intercourse. A few STDs, such as scabies, can also be transmitted without sexual intercourse via direct contact with an infected site of a sex partner. Other more important blood-borne pathogens, such as hepatitis B virus, human T-cell lymphotrophic virus type I, and HIV, are transmitted among adults not only by sexual intercourse, but also by parenteral routes—particularly among intravenous drug users through contaminated injecting drug equipment. The relative contribution of parenteral versus sexual transmission varies according to the risk behaviors of the population and other factors. In addition, pregnant women with an STD may pass their infection to infants in the uterus, during birth, or through breast-feeding.
Human papillomavirus is associated with the development of cervical and other genital and anal cancers (Koutsky et al., 1988; Reeves et al., 1989) and is prevalent across all socioeconomic groups in the United States. An estimated 24 million Americans already are infected with human papillomavirus, and as many as one million new human papillomavirus infections occur each year (CDC, DSTD/HIVP, 1995). In one study of female college students presenting for care at a university health center, genital human papillomavirus infections were five times more common than all other STDs combined (Laura Koutsky and King Holmes, University of Washington, unpublished data, 1995).