Biological risk factors for HIV

HIV is transmitted through bodily fluids. Biological features that increase an uninfected person's contact with infected body fluids will therefore increase that person's chances of contracting HIV. One such biological feature is having an STI/STD. In some studies, HIV-negative people with an STI/STD who had sex with an HIV-positive partner were 10 times more likely to contract HIV than were HIV-negative people without an STI/STD (Chin, 2000; Osmond, 1998).

People with HIV and STIs/STDs are also more likely to shed blood and pus through genital sores, exposing their sexual partners to HIV. In addition, the immune systems of people with STIs/STDs send immune cells to these genital lesions to fight the infections there. These immune cells–CD4+ cells–are precisely the cells to which HIV attaches. As a result, men and women with HIV and STIs/STDs have many more HIV particles concentrated near their penis and vagina, respectively, than do HIV+ people without STIs/STDs. For example, one study found that HIV+ men with STIs/STDs had 8 times more HIV in their semen than did HIV+ men without STIs/STDs (Osmond, 1998). Thus, having an STI/STD not only makes people with HIV more likely to expose uninfected partners to their bodily fluids, but it also makes their bodily fluids more contagious (ie, the fluids have more HIV particles).

Uninfected people may contract HIV through the cuts and scrapes that they get during vaginal intercourse. In particular, the biological risk of contracting HIV through heterosexual sex is greater for women than for men for several reasons (Volberding, 1998; World Health Organization [WHO], 2000):

• The vagina has larger areas of exposed, sensitive skin than does the penis
• HIV has an easier time surviving in the vagina than it does on the surface of the penis
• There are more copies of the virus in a man's semen than there are in the fluids of a woman's vagina

HIV also can be transmitted through the tearing that can take place during anal intercourse. In heterosexual anal intercourse, the biological risk of contracting HIV is greater for women than for men, as the anus has larger areas of exposed, sensitive skin than does the penis, and as the HIV is more likely to survive in the anus than on the penis (Volberding, 1998; WHO, 2000). Moreover, men's semen has more HIV than do women's anal fluids (WHO, 2000). For these same reasons, among men who have sex with men, receptive anal sex (ie, being the person who is penetrated by a partner) is riskier than insertive anal sex (Osmond, 2004).

Additionally, studies have shown that uncircumcised men are at greater risk of acquiring HIV and of transmitting HIV than circumcised men. The inner surface of the penis’s foreskin contains cells called Langerhans cells that are particularly receptive to the HIV virus. These cells are likely to be the primary point of viral entry into the penis of an uncircumcised man. Male circumcision, which surgically removes all or part of the foreskin, has been shown in several studies to reduce the likelihood that a man without HIV will become infected with HIV or that a man with HIV will transmit it. Male circumcision also can reduce the likelihood of infection with other STIs/STDs that can facilitate HIV transmission (CDC, 2006; Szabo & Short, 2000).

Higher concentrations of HIV are found in blood and semen during the first stage of HIV disease—that is, during acute infection—than at later stages of disease (Lawn, Butera, & Folks, 2001). The more HIV a person has (ie, the greater a person's viral load), the more likely he or she is to transmit HIV to someone else.