HIV is transmitted through sex when the virus, present in certain body fluids, crosses from one person’s body into another through mucous membranes or microscopic breaks in tissue. The fluids that carry enough virus to transmit it are semen (including pre-seminal fluid), vaginal fluid, rectal fluid, and blood. Not all sexual acts carry the same level of risk, and several factors can dramatically raise or lower the probability of transmission during any single encounter.
How the Virus Enters the Body
During sex, HIV needs to cross a barrier of skin or mucous membrane to reach vulnerable cells underneath. The lining of the rectum, vagina, cervix, and the opening of the penis all contain mucous membranes, which are thinner and more permeable than regular skin. The rectal lining is especially fragile because it’s only a single cell layer thick in places, which is why anal sex carries the highest per-act risk.
Once the virus crosses that outer barrier, it targets a specific type of immune cell. These are CD4+ T cells, the white blood cells HIV is best known for attacking. Specialized immune cells near the mucosal surface, called dendritic cells and Langerhans cells, may also pick up the virus and inadvertently deliver it to CD4+ T cells deeper in the tissue. From there, the virus begins replicating and spreading through the body. This initial foothold can involve multiple types of immune cells, which is part of why HIV is so effective at establishing infection.
Risk by Type of Sexual Act
The CDC estimates per-act transmission probabilities assuming no condoms, no PrEP, and no antiretroviral treatment. These numbers represent the risk per 10,000 exposures with an HIV-positive partner:
- Receptive anal sex: 138 per 10,000 acts (about 1.4%)
- Insertive anal sex: 11 per 10,000 acts (about 0.11%)
- Receptive vaginal sex: 8 per 10,000 acts (about 0.08%)
- Insertive vaginal sex: 4 per 10,000 acts (about 0.04%)
Receptive anal sex is the highest-risk sexual activity by a wide margin, roughly 17 times riskier per act than receptive vaginal sex. This is because the rectal lining is thin, has a rich blood supply, and is more prone to small tears during intercourse. The receptive partner in any act faces greater risk than the insertive partner because they have more mucosal surface exposed to infectious fluid for a longer period.
These are averages. Individual encounters can be far riskier or far safer depending on factors like viral load, the presence of other infections, and whether there are any cuts or sores.
Why Viral Load Matters So Much
The amount of virus in a person’s body fluids is the single biggest factor determining whether transmission occurs. Research from the University of Washington showed that for every tenfold increase in the concentration of HIV in semen, a man’s risk of transmitting the virus to a female partner increased 1.75-fold. For women, each tenfold increase in cervical viral levels raised the risk of transmitting to a male partner 2.2-fold.
This relationship works in the other direction too. When someone with HIV takes antiretroviral therapy and their viral load drops below 200 copies per milliliter of blood (classified as “undetectable”), the risk of sexual transmission drops to effectively zero. The landmark PARTNER study followed couples where one partner was HIV-positive and on treatment while the other was HIV-negative. Over thousands of acts of condomless sex, including both vaginal and anal intercourse, zero linked transmissions occurred. This is the basis of the widely cited principle: Undetectable = Untransmittable, or U=U.
The Danger of Early Infection
People are most contagious during the first weeks after contracting HIV, a period called acute or primary infection. During this stage, viral load skyrockets because the immune system hasn’t yet mounted a response. A person with acute HIV infection is estimated to be about 26 times more infectious than someone in the long, stable “chronic” phase of untreated infection.
This creates a particularly dangerous cycle. Most people with acute HIV don’t know they have it. They may feel fine or experience symptoms easily mistaken for the flu. Standard antibody-based HIV tests can miss acute infections because the body hasn’t produced enough antibodies yet. So the period when a person is most likely to transmit the virus is also the period when they’re least likely to know they have it or to show up on a standard screening test.
How Other STIs Increase Risk
Having another sexually transmitted infection at the same time more than doubles the risk of either acquiring or transmitting HIV. This applies to both partners: an STI in the HIV-negative person makes them more vulnerable, and an STI in the HIV-positive person makes them more infectious.
Ulcerative STIs like syphilis and herpes are especially dangerous. They create open sores that give HIV a direct path through the skin barrier, and they trigger an immune response that concentrates exactly the type of cells HIV targets right at the site of the sore. Studies have found that ulcerative STIs can increase HIV risk anywhere from 2 to 11 times, while non-ulcerative infections like gonorrhea and chlamydia raise risk by a factor of 3 to 4. Non-ulcerative STIs increase HIV shedding in semen, urethral fluid, and cervical fluid, meaning an HIV-positive person with gonorrhea or chlamydia has more virus in their genital secretions than they otherwise would.
What Reduces the Risk
Several tools can significantly lower the chance of HIV transmission during sex, and they work even better in combination.
Antiretroviral treatment (U=U): As described above, an HIV-positive person on effective treatment with an undetectable viral load does not transmit the virus sexually. This is the most powerful prevention tool available.
PrEP: Pre-exposure prophylaxis is a medication taken by HIV-negative people before potential exposure. When taken consistently as prescribed, PrEP reduces the risk of getting HIV from sex by about 99%. It’s available as a daily pill or as a long-acting injection given every two months.
Condoms: Consistent and correct use of latex condoms reduces heterosexual HIV transmission by an estimated 87%, with effectiveness ranging from about 60% to 96% depending on how reliably they’re used. The wide range reflects the difference between perfect use and typical use. Condoms also protect against other STIs, which indirectly reduces HIV risk further.
Male circumcision: Medical male circumcision reduces the risk of female-to-male HIV transmission by approximately 60%. The foreskin contains a high density of the immune cells HIV targets, so its removal reduces the available entry points. This protective effect applies specifically to the insertive partner during vaginal sex and does not directly protect female partners or receptive partners in anal sex.
Combining methods provides the strongest protection. A person using PrEP who also uses condoms with a partner on effective treatment faces a risk so close to zero that it’s essentially immeasurable.