the course of the infection (Burke and Redfield, 1988). Researchers from NIDA recently have argued that the relative frequency of female-to-male transmission has been underestimated (primarily as a result of the way that cases are classified), suggesting that it represents a more significant public health concern than is generally believed (Haverkos and Battjes, 1992).

In sum, although many highrisk practices and cofactors have been observed most predominantly among men who have sex with men, they are not limited to that population. Nor are they the only behaviors linked to sexual risk. As the AIDS epidemic spreads more widely among women and heterosexual men, a broader range of sexual practices must be considered for their potential in transmitting HIV, including the possibility of female-to-female sexual transmission, which has been notably understudied. Although lesbians and bisexual women have been considered to be a low-risk group for contracting and transmitting HIV, the fact that women can transmit STDs such as chlamydia, herpes, and genital warts to one another makes it likely that they could similarly transmit HIV. There is some evidence that among injection drug users, women who have sex with women are at particular risk for HIV (Friedman, Des Jarlais, Deren, et al., 1992; Jose et al., 1993; Reardon et al., 1992).

The sharing of hypodermic needles, syringes, and other injection paraphernalia is the most likely route of HIV transmission among intravenous and other injection drug users. When the blood of the previous user is lodged in the needle, the syringe, or some other part of the works (drug paraphernalia), that blood serves as the vector for transmission. Levels of risk may vary depending on the particular injection practice, but this has not been well studied. One practice is skin-popping—the subcutaneous or intramuscular injection of cocaine, narcotics, and other drugs. Skin-popping (or simply popping ) is a common method of heroin use by experimenters and novice and casual users who mistakenly believe that addiction cannot occur through this route (Baden, 1975; Kaplan, 1983). Skin-popping also is done by long-term injectors who can no longer find veins into which they can inject.

Another injection method is booting, also known as kicking, a process that uses a syringe to draw blood from the user's arm, mixes the drawn blood with the drug already taken into the syringe, and injects the blood-drug mixture into the vein. Booting