insights into the HIV/AIDS epidemic in the United States. This chapter reviews these data with particular emphasis on the role of injection drug users. However, before reviewing these data, the panel thought it critical to provide the reader with a brief review of current knowledge of the underlying biological mechanisms involved in the transmission of the virus. The details of these behaviors and processes are important to developing an appreciation for the complexity of the issues at hand.

Although the consensus among the research community is that the development of an effective vaccine for the human immunodeficiency virus (HIV) is still years away, significant strides have been made in biomedical research. As Rogers (1992:522) stated, ''We now know quite precisely how the virus is transmitted and how it is not and what it does to human cells and the immune mechanism, and we know enough about its structure and life-cycle to have identified multiple potential points to get at it."

HIV transmission is limited to sharing of contaminated injection drug paraphernalia, sexual contact, transmission from infected mother to child, exposure to infected blood or blood products, and transplantation of infected organs or tissues. As of December 31, 1993, injection drug use and sexual contact accounted for approximately 92 percent of all adult and adolescent AIDS cases reported to CDC. We review here postulated mechanisms for transmission through activities associated with injection drug use, sexual, and perinatal transmission and detail the associated human behaviors.

Injection drug use involves practices that facilitate the transmission of HIV infection. The primary category of such practices is direct needle sharing, which involves the reuse of needles and syringes that have been contaminated through prior use by an infected individual. Penetration of the needle through the skin is sufficient for contamination and subsequent transmission of HIV infection, as has been demonstrated in cases of needlestick injuries among health care workers (McCray, 1986). In instances of occupational exposure of health care workers, in which the amount of blood exposure frequently is small (Napoli and McGowan, 1987), the risk of transmission is about 3/1,000 exposures (Ippolito et al., 1994).

The higher rates of HIV infection in injection drug users than in health