Can you get HIV from one time?

One time can be enough, but usually isn’t

HIV risk is measured per act. The figures below are averages from large studies. They don’t mean you get a certain number of “free” encounters: transmission could happen the first time, or not at all over many acts. But for any single act, the chance is small.

Risk from one act with a partner who has HIV

These estimates assume the other person has HIV and is not on effective treatment, and that no condom, PrEP or PEP was used:

• Receptive anal sex (bottom): 1.4% (about 1 in 72)
• Insertive anal sex (top): 0.11% (about 1 in 910)
• Receptive vaginal sex: 0.08% (about 1 in 1,200)
• Insertive vaginal sex: 0.04% (about 1 in 2,500)
• Oral sex: 0.01% (about 1 in 10,000)
• Sharing needles to inject drugs: 0.63% (about 1 in 160)
• Needlestick injury: 0.23% (about 1 in 430)

Receptive anal sex and sharing needles carry the highest risk. Oral sex carries very little. Kissing, hugging and sharing food or drinks carry none, because HIV isn’t passed on through saliva, sweat or tears. See how HIV is not transmitted.

When you don’t know their status

Most one-time encounters are with someone whose HIV status isn’t known. Then your risk depends on two things: how likely that person is to have HIV, and how risky the act was. Because most people don’t have HIV, the result is usually far lower than the figures above.

Here is the estimate for a man having vaginal sex once, without a condom, with a woman of unknown status in different countries:

For a woman having vaginal sex with a man of unknown status, the estimate is 0.00014% (about 1 in 730,000) in the United States and 0.0023% (about 1 in 44,000) in South Africa. For a man bottoming with a man of unknown status in the United States, it’s 0.05% (about 1 in 2,000). That’s higher because HIV is more common among men who have sex with men.

What makes one time riskier

• A partner who was recently infected. Viral load peaks in the first weeks, often before the person knows. The calculator raises the estimate 10 times for this.
• Another STI. Sores and inflammation make it easier for HIV to get in. The calculator multiplies the estimate by 2.
• Ejaculation inside you. If a male partner didn’t ejaculate inside you, the calculator lowers the estimate by 50%.

What brings it down, sometimes to zero

• Treatment. A partner with HIV who keeps an undetectable viral load doesn’t pass HIV on through sex. That’s U=U.
• Condoms. About 80% protection in real-world use. If one failed, see condom broke.
• PrEP. PrEP (pre-exposure prophylaxis) is medicine that people without HIV take to prevent it. Taken as prescribed, it lowers the risk from sex by about 99% for anal sex and about 90% for vaginal sex.
• PEP. Started within 72 hours, it can stop an infection from taking hold.

What to do now

1. If it was within 72 hours, and your partner has HIV or you don’t know their status, go for PEP. Don’t wait for symptoms or a test. PEP is taken daily for 28 days.
2. If it was more than 72 hours ago, PEP is no longer recommended. Focus on testing at the right time.
3. Test after the window period. A lab antigen/antibody test (blood from a vein) can detect HIV from 18 days, and a negative result is conclusive from 45 days. A NAT (nucleic acid test) can detect HIV from 10 days. If you take PEP, expect follow-up tests about 4 to 6 weeks and 12 weeks after the exposure. Our test accuracy by week guide has the details.
4. Look after yourself while you wait. Worry after a single encounter is very common, and the numbers are usually reassuring. Our page on HIV anxiety may help.