Transmission risk by activity
A comparative view of which activities carry which risks, with the numbers behind them.
How to read these numbers
The figures below are pooled per-act estimates for HIV from an untreated partner with a detectable viral load, drawn from CDC's synthesis and the systematic review by Patel et al. (2014, AIDS). They are averages across populations, not predictions for an individual encounter, and confidence intervals around several are wide. They rise substantially with genital ulcers, other untreated STIs, high viral load during acute infection, menstruation, and mucosal trauma. They fall to effectively zero when the partner with HIV has a sustained undetectable viral load.
Per-act HIV risk, receptive and insertive
| Activity | Estimated risk per 10,000 exposures |
|---|---|
| Receptive anal intercourse | 138 |
| Insertive anal intercourse | 11 |
| Receptive vaginal intercourse | 8 |
| Insertive vaginal intercourse | 4 |
| Receptive oral sex (on a penis) | Low; estimated 0–4 |
| Insertive oral sex | Low; near 0 |
| Oral–vulval or oral–anal contact | Negligible for HIV |
| Needle sharing | 63 |
| Blood transfusion (unscreened) | 9,250 |
Two points people consistently misread. First, insertive partners are not safe — 11 per 10,000 for insertive anal is a real, cumulative risk. Second, low per-act risk compounds: the same small number repeated hundreds of times is not a small number.
Other infections do not follow the HIV ranking
HIV is one of the least transmissible of the common STIs per act. Activities that are low-risk for HIV are frequently high-risk for others.
- Oral sex carries meaningful risk for gonorrhoea (pharyngeal, usually asymptomatic), syphilis, herpes (HSV-1 to genitals is now the leading cause of new genital herpes in many high-income countries), HPV and hepatitis A and B.
- Skin-to-skin contact transmits herpes, HPV, syphilis, molluscum, mpox, pubic lice and scabies. Condoms reduce but cannot eliminate these, because they cover only part of the exposed area. Winer et al. (2006, NEJM) found consistent condom use reduced genital HPV acquisition by about 70 percent — a real reduction, and not complete protection.
- Shared toys transmit HPV, herpes, trichomoniasis, hepatitis and bacteria unless a condom is changed between users or the toy is properly cleaned.
- Fingering and fisting can transmit HPV, herpes and bacteria, and can cause tissue trauma that raises risk for everything else. Gloves and lubricant address both.
- Rimming is a well-documented route for hepatitis A, shigella, giardia, E. coli and intestinal parasites, alongside herpes and HPV. Hepatitis A vaccination is the specific protective measure.
What lowers risk, in rough order of effect
- Treatment as prevention. Undetectable viral load, zero sexual HIV transmission (PARTNER2).
- PrEP. Around 99 percent reduction in HIV acquisition when taken as prescribed.
- Vaccination. HPV, hepatitis A, hepatitis B, mpox. These prevent infections that no barrier fully blocks.
- Barriers used consistently. Around 80 percent HIV reduction with consistent use in Cochrane review data; substantial reduction for gonorrhoea, chlamydia and HIV; partial for HPV and herpes.
- Regular three-site testing and prompt treatment. Untreated STIs raise HIV transmission risk in both directions through mucosal inflammation.
- Lubricant. Reduces microtrauma, the mechanism behind much transmission.
- Suppressive antivirals for a partner with herpes — roughly 48 percent reduction in transmission (Corey et al., 2004).
- Sequencing acts so that higher-risk activity does not follow tissue trauma; not moving from anus to vagina or mouth without changing barriers.
What does not lower risk
- Withdrawal. It does not prevent STIs and is unreliable for pregnancy, since pre-ejaculate can contain motile sperm.
- Douching. Increases mucosal damage and is associated with higher, not lower, infection rates.
- Choosing partners by appearance, class, profession, education or apparent health.
- "Being clean." There is no such state, only a test date and what it covered.
- Doubling condoms, which increases breakage.
Risk is a decision, not a verdict
Everyone chooses a level of risk they will accept, and the level differs by activity, by partner, and over time. Informed choice means knowing the actual numbers, knowing which tools apply to which infection, and being honest with partners about what you are and are not doing. It does not mean eliminating risk.
Sources
- Patel P et al. "Estimating per-act HIV transmission risk: a systematic review." AIDS, 2014.
- CDC. "Estimated per-act probability of acquiring HIV from an infected source, by exposure act." Current edition.
- Rodger AJ et al. PARTNER2, Lancet, 2019.
- Winer RL et al. "Condom use and the risk of genital human papillomavirus infection in young women." NEJM, 2006.
- Corey L et al. "Once-daily valacyclovir to reduce the risk of transmission of genital herpes." NEJM, 2004.
- Weller SC, Davis-Beaty K. Condom effectiveness Cochrane review.
- UKHSA and ECDC. Shigella and hepatitis A outbreak guidance among men who have sex with men, current editions.
Last reviewed: August 2026.
Educational reference, not medical advice. Every article is community-edited and cites its sources; verify anything that affects your health with a clinician who knows your history.