References UK clinical guidance from the NHS, the British HIV Association (BHIVA), and the British Association for Sexual Health and HIV (BASHH). Last updated: April 2026.
The likelihood of contracting HIV varies enormously depending on the type of exposure, the HIV-positive partner’s viral load, whether protection is used and other factors. For many real-world scenarios the risk is very low, but it’s not zero. Understanding the numbers helps put fears and decisions in perspective.
Here’s an honest look.
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Per-Act Risk Estimates
These figures assume the source is HIV-positive with a detectable viral load and no condom or PrEP use:
| Activity | Estimated Risk per 10,000 Exposures |
|---|---|
| Receptive anal sex | Around 138 |
| Insertive anal sex | Around 11 |
| Receptive vaginal sex | Around 8 |
| Insertive vaginal sex | Around 4 |
| Sharing injecting equipment | Around 63 |
| Needlestick injury | Around 23 |
| Receptive oral sex | Very low — not reliably quantified |
What Reduces Risk
- Undetectable viral load (U=U) — zero sexual transmission
- PrEP — over 99% reduction when taken as prescribed
- Condoms — around 80-95% reduction when used correctly
- Circumcision — reduces female-to-male vaginal transmission
- No ulcerative STIs — reduces biological vulnerability
What Increases Risk
- High viral load in source partner
- Recent HIV infection (acute stage)
- Coexisting STIs (especially ulcerative ones)
- Bleeding during sex
- Lack of protection
- Receptive anal sex
Putting Numbers in Context
A risk of 138 per 10,000 sounds small, but over many exposures the odds add up. For example, regular condomless receptive anal sex with a partner with detectable HIV accumulates significant risk over time. This is why PrEP and U=U are so valuable.
Cumulative Risk
Each exposure is independent, but the overall chance of infection rises with:
- Number of exposures
- Number of different partners
- Partner HIV prevalence in your network
- Viral load and protection status of partners
Low-Risk Scenarios
- One-off condom-protected sex with a partner of unknown status — very low
- Sex with a partner on effective HIV treatment (U=U) — zero
- Oral sex on an HIV-positive partner — very low
- Mutual masturbation — essentially zero
When to Worry
- Condomless sex with a partner you believe has untreated HIV
- Sharing injecting equipment with someone of unknown status
- Sexual assault
- Needlestick injury with visible blood
In these situations, PEP within 72 hours can prevent infection.
How to Lower Your Risk
- Consider PrEP if you’re at ongoing risk
- Use condoms consistently
- Get tested regularly
- Know your status
- Ask partners about testing and U=U status
- Avoid sharing injecting equipment
Read more: Estimating HIV risk | PrEP | U=U for partners
Frequently Asked Questions
Can I catch HIV from one exposure?
Yes, though it’s statistically uncommon per act. Risk depends on circumstances.
Is HIV risk the same for everyone?
No. Risk depends on activity, protection, viral load and other factors specific to each encounter.
How can I get certainty after a risky exposure?
Test at 4-6 weeks (4th-generation test) and again at 3 months for definitive results.
Related reading
- A to Z of Sex: Sexual Health Terms Explained
- Alcohol and HIV: What You Need to Know
- Anxiety and HIV: Coping and Getting Support
- Are All STIs Curable?
- Blood Problems and HIV: Anaemia, Low Platelets and More
Sources
- NHS — Sexual health conditions
- BASHH — UK clinical guidelines for STI management
- British HIV Association (BHIVA) — HIV treatment guidelines
- Terrence Higgins Trust — sexual health information
- UK Health Security Agency — STI surveillance data
Steve Page is a recognised expert on Sexually Transmitted Diseases (STDs) and STD treatments, having published numerous articles in peer-reviewed journals and presented his research at conferences around the world. He has an in-depth understanding of the latest medical research on STDs, and is an advocate for the development of new treatments and protocols to improve the health of those affected. In addition to his research, he has dedicated his career to understanding the causes and symptoms of STDs, as well as how to best treat those impacted.
