The chances of getting HIV from a single sexual encounter depend heavily on the type of sex, whether protection is used, and the HIV status and treatment of the partner involved. Without any form of protection, the highest-risk activity is receptive anal sex, with roughly a 1.4% chance per act (138 per 10,000 exposures). The lowest-risk penetrative activity is insertive vaginal sex, at about 0.04% per act (4 per 10,000). These numbers come from the CDC’s HIV Risk Reduction Tool and assume no condoms, no PrEP, and no antiretroviral treatment.
Risk by Type of Sexual Act
Not all sex carries the same HIV risk. The rectal lining is thin and prone to small tears during intercourse, which is why anal sex accounts for the highest per-act transmission rates. Here’s how the numbers break down per 10,000 exposures with an HIV-positive partner who is not on treatment:
- Receptive anal sex: 138 per 10,000 (about 1 in 72)
- Insertive anal sex: 11 per 10,000 (about 1 in 909)
- Receptive vaginal sex: 8 per 10,000 (about 1 in 1,250)
- Insertive vaginal sex: 4 per 10,000 (about 1 in 2,500)
Oral sex carries little to no risk. For transmission to happen through oral contact, something unusual would need to be present, like open sores, bleeding gums, or ejaculation in the mouth combined with oral ulcers. While oral sex is not zero-risk in theory, documented cases of transmission through this route alone are extremely rare.
These per-act numbers can be misleading if you look at them in isolation. A 1-in-1,250 chance per encounter sounds small, but risk accumulates over repeated exposures. Someone having unprotected receptive vaginal sex regularly with an HIV-positive partner who isn’t on treatment faces meaningfully higher cumulative odds over months or years.
Factors That Raise the Risk
The per-act estimates above are averages. Several biological factors can push the actual risk higher during any given encounter.
Having another sexually transmitted infection is one of the most significant risk multipliers. Infections like syphilis, herpes, or gonorrhea cause inflammation or open sores in the genital or rectal area, creating easier entry points for the virus. This works in both directions: STIs in the HIV-negative partner make it easier to acquire HIV, and STIs in the HIV-positive partner can increase the amount of virus present in genital fluids.
The stage of the HIV-positive partner’s infection also matters enormously. During acute infection, the first few weeks after someone contracts HIV, viral levels in blood and sexual fluids spike dramatically. A person in this phase is far more infectious than someone who has been living with chronic, stable HIV. This is especially concerning because most people in acute infection don’t yet know they have the virus.
Factors That Lower the Risk
The same numbers that describe unprotected sex with an untreated partner drop sharply when prevention tools are in play. Condoms, when used correctly, reduce HIV transmission risk by a large margin for every type of sexual contact.
PrEP, a preventive medication taken by HIV-negative people, reduces the risk of getting HIV from sex by about 99% when taken as prescribed. It’s available as a daily pill or a long-acting injection given every two months. For someone concerned about ongoing exposure, PrEP effectively transforms even higher-risk activities into very low-risk ones.
Male circumcision provides partial protection specifically for the insertive partner during vaginal sex. Randomized trials found it lowers the risk of female-to-male transmission by approximately 60%. It does not eliminate risk, and it offers no documented protection for receptive partners or during anal sex.
Undetectable Means Untransmittable
Perhaps the single most important fact for anyone worried about HIV from a sexual partner: a person living with HIV who takes treatment and maintains an undetectable viral load has zero risk of transmitting the virus sexually. This is not a reduction in risk. It is zero. The CDC, the WHO, and every major HIV organization endorse this principle, commonly known as U=U (Undetectable = Untransmittable).
This means that if your partner is HIV-positive, on treatment, and has lab results showing an undetectable viral load, the per-act risk numbers listed above do not apply. The effective transmission risk drops to zero regardless of the type of sex, whether condoms are used, or how many times you have sex. Consistent treatment and regular viral load monitoring are what make this possible.
What to Do After a Possible Exposure
If you’ve had unprotected sex with someone who may have HIV, a medication called PEP (post-exposure prophylaxis) can prevent infection after the fact. PEP must be started within 72 hours of exposure, and ideally as soon as possible. It involves taking antiretroviral medication for 28 days. Observational research suggests PEP reduces the risk of acquiring HIV by more than 80%, and the actual effectiveness is likely higher when taken consistently as prescribed. Emergency rooms and sexual health clinics can prescribe it.
After any potential exposure, testing is the only way to know your status. The timing of the test matters because HIV doesn’t show up immediately. A lab-based blood draw (antigen/antibody test from a vein) can usually detect HIV between 18 and 45 days after exposure. A rapid finger-stick test has a wider window, typically detecting HIV between 18 and 90 days. Testing before these windows close can produce a false negative, so a follow-up test at the appropriate interval is important if your initial result is negative.
Putting the Numbers in Context
Per-act transmission probabilities are useful for understanding relative risk, but they don’t capture the full picture of any individual encounter. You likely don’t know your partner’s viral load, whether they have an undiagnosed STI, or whether they’re in the acute phase of infection. The averages flatten all of these variables into a single number.
What the data does make clear is that the tools to prevent HIV work extraordinarily well. Condoms reduce risk substantially. PrEP reduces it by 99%. An undetectable viral load eliminates it entirely. The per-act numbers that look worrying in a vacuum become vanishingly small when even one of these layers of protection is in place, and nearly nonexistent when multiple layers overlap.