HIV in Heterosexual Couples

There is always a risk of transmission and acquisition of HIV in condom-less sex, for both women and men. While the risk tends to be higher among women due to biologic vulnerability (including the greater mass of mucosal tissues that HIV can breach), men are also at risk, with everything from concurrent sexually transmitted diseases (STDs) to circumcision status adding to that risk.

Transmission Risk in Women

HIV can be found in the blood, semen, pre-seminal fluid ("pre-cum"), or vaginal fluid of a person infected with the virus. It can also be found in rectal fluid and breast milk. It can be transmitted through small tears in the mouth, throat, rectum, or lining of the vagina, allowing HIV to enter the body. HIV can also be absorbed through the mucous membranes that line the vagina and cervix.

When HIV is exposed to these tissues, the localized infection is attacked by first-line immune cells, including macrophage and dendritic cells. The activation of these cells sparks a reaction from the immune system in which specialized defensive cells, including CD4 and CD8 T-cells, are spurred to the fight. Ironically, it is the CD4 cells that HIV preferentially targets for infection. If this occurs and the first-line defenses are unable to contain the HIV invaders, an exposure becomes more than just exposure. It becomes an infection.

Transmission Risk in Men

Though a male is at somewhat less risk for HIV than a female, HIV can enter either through his urethra (the opening at the tip of the penis) or through small cuts or open sores on the penis. Anal sex increases the risk of HIV transmission as well.

Additionally, men who are uncircumcised tend to be more vulnerable to HIV than men who are circumcised. The bacterial population that exists beneath a foreskin can thrive due to a moist environment. The immune system naturally responds by triggering a modest immune defense to keep an infection at bay. Again, ironically, CD4 cells can be called to the front lines of defense, making transmission all the easier.

Sexually Transmitted Infections

Sexually transmitted infections (STIs) work more or less in the same way. Beyond open ulcerative sores that can facilitate an easy route into the bloodstream (from STIs like syphilis or herpes simplex), other infections spur a localized immune response, exponentially increasing the likelihood of HIV transmission or acquisition well beyond what might occur if there were no STI.

Preventing HIV Transmission

If you choose to have vaginal intercourse or oral or anal sex, use a latex condom to protect both you and your partner from the risk of HIV and other STDs. Studies have shown that latex condoms are very effective when used correctly and consistently. If either partner is allergic to latex, plastic (polyurethane) condoms made for either men or women can be used. (Avoid lambskin condoms, which do not offer protection from HIV and STDs).

Additionally, partners can significantly reduce HIV with two additional preventive strategies:

• An HIV-positive partner, male or female, should be placed on antiretroviral therapy to achieve full suppression of the virus to undetectable levels. If this is achieved, and the partner has sustained viral suppression for at least six months, the risk of transmission to an uninfected partner can drop to zero. This is called HIV treatment as prevention (TasP).
• Any HIV-negative partner, male or female, can opt to take HIV pre-exposure prophylaxis (PrEP)—a type of therapy (either in the form of a daily pill or an injection every two months) that can reduce the risk of HIV from sex by about 99%.