HIV has been found in the semen, blood, vaginal secretions, saliva, urine, and breast milk of infected individuals. It also can occur in any other bodily fluids that contain blood, including cerebrospinal fluid and amniotic fluid. Blood, semen, and vaginal secretions are the three bodily fluids that most consistently contain high concentra­tions of the virus in infected people. Most commonly, HIV enters the body when bodily fluids are exchanged during unprotected vaginal or anal intercourse with an infected person. Transmission of HIV through sexual contact is estimated to be the cause of about 80% of worldwide HIV infections. HIV is also readily transmitted by means of blood-contaminated needles shared by injection drug users.

The virus can also be passed perinatally from an infected woman to her fetus before birth, to her infant during birth, or to her baby after birth through breast-feeding (Kumwenda et al., 2008; Osborn, 2008). Mother-to-child transmission (MTCT) is the primary way that children are infected with HIV.

The likelihood of transmitting HIV during sexual contact depends on both the viral dose and the route of HIV exposure. Viral dose is a direct effect of the viral load— how much virus is present in an infected person’s blood. The viral load measurement widely used is the number of individual viruses in a milliliter of blood. In general, the greater the viral load, the higher the chance of transmitting the infection. As common sense would suggest, when a person is in a late stage of HIV/AIDS disease, with more advanced infection and thus greater viral load, he or she is highly infectious. However, many readers might be surprised to learn that evidence strongly indicates that in the initial period between exposure to HIV and the appearance of HIV antibodies in the blood—a period called primary infection, which usually lasts a few months—viral load can be extremely high, creating a state of heightened infectiousness (Harris & Bolus, 2008; Shapiro & Ray, 2007). This relatively brief peak in the transmissibility of HIV soon after a person is infected is especially troubling because most infected people are likely to remain unaware during these few months that they have been invaded by HIV. Some experts believe that transmission during primary infection accounts for a large portion of HIV infections worldwide (Cohen & Pilcher, 2005; Wawer et al., 2005).

The likelihood of infection during sexual activity is greater when HIV is transmit­ted directly into the blood (e. g., through small tears in the rectal tissues or vaginal walls) rather than onto a mucous membrane. Researchers have become increasingly aware that circumcision status affects a man’s risk for contracting HIV. The foreskin of the uncir­cumcised penis is soft and prone to tiny lacerations that may allow HIV to enter the bloodstream more easily. In addition, the foreskin has high concentrations of CD4 and Langerhans cells, the immune cells typically targeted by HIV (Reynolds et al., 2004; Seppa, 2005). While health-care providers continue to debate the practice of circumci­sion on medical and ethical grounds, the case for circumcision as a means for reducing HIV transmission is building. Evidence supporting this position is discussed in the Spotlight on Research box, "Circumcision as a Strategy for Preventing HIV Infection."

Research also suggests that HIV can be transmitted during oral sex when the virus present in semen or vaginal secretions comes into contact with mucous membrane tis­sues in the mouth. Unfortunately, many people mistakenly consider oral sex to be a safe practice (Kaestle & Halpern, 2007). Current CDC recommendations for preventing HIV transmission call for using a condom during mouth-to-penis contact. However, it is rare for people to use condoms during oral sex (Torassa, 2000). If you engage in unprotected oral sex with partners whose HIV status is unknown, it would be wise to take certain precautions: Make sure that your gums are in good shape (oral sores or breaks in gum tissue provide HIV easier access to blood), avoid flossing immediately before or after sex (flossing can damage oral tissue and cause bleeding), and avoid taking

SPOTLIGHT ON

RESEARCH

ejaculated semen into your mouth. Furthermore, in light of the often substantial con­centration of HIV in vaginal fluids, you might also be cautious about engaging in cun – nilingus with a female partner who has not tested negative for HIV. Finally, even though there is some risk of HIV transmission via oral sex, the current consensus of experts is that unprotected oral sex is an effective risk reduction strategy compared to unprotected vaginal or anal penetration (Shapiro & Ray, 2007). •

In the early 1980s, before the U. S. government required screening of donated blood for HIV, contaminated blood and blood products infected an estimated 25,000 transfusion recipients and people with blood-clotting disorders (such as hemophilia) in the United States (Graham, 1997). However, since early 1985, donated blood and blood products have been screened with extensive laboratory testing for the presence of HIV antibod­ies. "The risk of transfusion transmitted HIV infections has been almost eliminated by

Sexually Transmitted Infections

the use of questionnaires to exclude donors at higher risk for HIV infection and the use of highly sensitive laboratory screening to identify infected blood donations" (Centers for Disease Control, 2010f, p. 1335). There is no danger of being infected as a result of donating blood. Blood banks, the Red Cross, and other blood-collection centers use sterile equipment and a new disposable needle for each donor. Unfortunately, U. S. pro­cedures for safeguarding the blood supply are not widely practiced globally. This problem is especially acute in some of the world’s poorest nations, which also have high rates of blood-transmitted diseases, such as HIV and viral hepatitis (Lamptey et al., 2006).

Research indicates that a small percentage of people appear to be resistant to HIV infection and that about 1 in 300 untreated HIV-infected people do not progress to AIDS (Collins & Fauci, 2010; Lok, 2011). Evidence suggests that in some individu­als this resistance has a genetic basis. Research in the laboratory of Stephen O’Brien (2003), a respected medical geneticist, has indicated that people who inherit two cop­ies of a gene labeled CCR5-32, one from each parent, are resistant to HIV infection. CCR5 is a protein receptor on the surface of CD4 cells that acts as a docking station for HIV. People who are homozygous for the CCR5-32 gene—about 1% of White Ameri­cans—lack this docking station (HIV’s doorway to cellular infection) and therefore are resistant to infection. This gene is much less common among African Americans, and the few copies of CCR5-32 among this population "derive exclusively from Caucasian gene flow to the African slaves and their descendants since their transport to America" (O’Brien, 2003, p. 215). This protective gene is completely absent in native African and native East Asian ethnic groups.

It is believed that the risk of transmitting HIV through saliva, tears, and urine is extremely low. Furthermore, no evidence indicates that the virus can be transmitted by casual contact, such as hugging, shaking hands, cooking or eating together, or other forms of casual contact with an infected person. All the research to date confirms that it is sexual contact with an infected person or sharing contaminated needles that places an individual at risk for HIV infection. Furthermore, certain high-risk behaviors increase the chance of infection. These behaviors include having multiple sexual part­ners, engaging in unprotected sex, having sexual contact with people known to be at high risk (such as injection drug users, sex workers, and people with multiple sexual partners), sharing drug injection equipment, and using non – injected drugs such as cocaine, marijuana, and alcohol, which can impair good decision making.