PCP HIV AIDS Toolkit/HIV Transmission/Handout A: Biological Risk Factors

PCP HIV AIDS Toolkit Handout A: Biological Risk Factors
This page is part of the PCP HIV AIDS Toolkit.

Handout A: Biological Risk Factors of HIV Transmission


Presence of sexually transmitted infections (STIs)


Sexually transmitted infections (STIs) increase one’s chances of becoming infected with HIV, and can also indicate high-risk sexual behavior, which facilitates the transmission of HIV infection.

  • STIs that cause ulcerative genital sores (such as herpes, syphilis, and chancroid) create an easy entry for HIV to enter and infect the body.
  • Non-ulcerative STIs (such as gonorrhea and chlamydia) can cause microscopic breaks in tissue, which are associated with HIV transmission.
  • Lymphocytes (the types of blood cells most likely to become infected if exposed to HIV) have a tendency to concentrate in the genital tract of people with STIs. This makes someone with an STI both more likely to transmit HIV and more vulnerable to it.
  • In people infected with HIV, the additional presence of an STI can increase the amount of virus in their genital secretions, which can make it more likely that their sexual partners will become infected with HIV.

Type of Sexual Intercourse


Anal Intercourse


Anal sex, the insertion of a male’s penis into a person’s anus (male or female), is one of the riskiest forms of sexual activity in terms of getting or giving an STI, including HIV. In general, the person receiving the se men is at greater risk of getting HIV because the lining of the rectum is thin and easily lacerated, and may allow the virus to enter the body during anal sex. However, a person who inserts his penis into an infected partner is also at risk because HIV can enter through the urethra (the opening at the tip of the penis) or through small cuts, abrasions, or open sores on the penis.

Vaginal Intercourse


Vaginal intercourse, the insertion of a male’s penis into a female’s vagina, is a high-risk sexual activity for both the male and female. However, certain biological factors can put the female at particular risk of HIV infection:

  • There is greater exposed surface area in the female genital tract than the male’s.
  • There are higher concentrations of HIV in semen than in vaginal fluids.
  • A larger amount of semen is exchanged during intercourse than vaginal fluids.
  • Coercive or forced sex may lead to abrasions in the vagina that facilitate entry of the virus.
  • Young girls are at higher risk for rips and tears in vaginal lining and cervical wall during intercourse.

Oral sex


Numerous studies have demonstrated that oral sex, which involves giving or receiving oral stimulation (e.g., sucking or licking) to the penis, the vulva, and/or the anus, can result in the transmission of HIV and other STIs. However, the risk of HIV transmission through oral sex is smaller than the risk of HIV transmission from anal or vaginal sex. Several co-factors can increase the risk of HIV transmission through oral sex, including: oral ulcers, bleeding gums, genital sores, and the presence of other STIs. HIV can be transmitted through exposure of the mucous membranes of the mouth (especially if the tissue is cut or torn), to vaginal secretions, menstrual blood, and semen. The potential for transmission is greater during early and late-stage HIV infection, when the amount of virus in the blood is expected to be highest.

Male circumcision


Male circumcision is the surgical removal of some or all of the foreskin (or prepuce) from the penis. Results from recent studies in Africa have demonstrated that male circumcision significantly reduces the risk of men becoming infected with HIV. Biologically, the inner mucosa of the foreskin has a higher density of target cells for HIV infection, and has been shown to be more susceptible to HIV infection in laboratory studies compared to the dry external skin surface. The foreskin may also have greater susceptibility to tears during intercourse, providing a portal of entry for HIV.

Ecologic studies have also indicated a strong association between lack of male circumcision and HIV infection at the population level. Although links between circumcision, culture, religion, and risk behavior may account for some of the differences in HIV-infection prevalence, the countries in Africa and Asia with prevalence of male circumcision of less than 20 percent have an HIV-infection prevalence several times higher than countries in those regions where more than 80 percent of men are circumcised.

Male circumcision does not provide complete protection against HIV infection. Circumcised men can still become infected with the virus and, if HIV-positive, can infect their sexual partners. Male circumcision should never replace other known effective prevention methods.

Further studies are underway to assess the impact of male circumcision on the risk of HIV transmission to female partners.

For more information on male circumcision, please refer to: