Each act is its own chance

Per-act figures are averages. With a partner who has HIV and isn’t on treatment, one act of receptive vaginal sex is estimated at 0.08% (about 1 in 1,200). Repeat it, and the chances combine:

  • 10 acts: 0.8% (about 1 in 130)
  • 100 acts: 7.7% (about 1 in 13)

For receptive anal sex, the riskiest type of sex, the same partner gives 1.4% (about 1 in 72) for one act and 13% (about 1 in 8) for ten.

The total is a little less than the per-act figure times the number of acts, because each act is a separate chance and infection can only happen once. It isn’t a countdown either: transmission could happen the first time, or not at all over many acts.

One regular partner vs different partners

When you don’t know a partner’s status, there are two questions: does this person have HIV, and did it pass on? How those combine depends on whether it’s one person or many.

With one regular partner, their status is either positive or negative, and it’s the same every time. If they don’t have HIV, no number of acts can pass it on. So the risk climbs at first, then levels off, and it can never exceed the chance that this one person has HIV.

With different partners, each new person is another draw: another chance that someone has HIV. The risk keeps rising with each new partner.

For a man having vaginal sex without a condom with women of unknown status in the United States:

  • One act: <0.0001% (about 1 in 1,500,000)
  • 100 acts with the same partner: 0.0067% (about 1 in 15,000)
  • 100 acts with different partners: 0.0069% (about 1 in 15,000)

And for a man bottoming without a condom with men of unknown status in the United States:

  • 10 acts with the same partner: 0.47% (about 1 in 210)
  • 10 acts with different partners: 0.5% (about 1 in 200)

Over these numbers of acts the two are close, because each act carries a small risk. The gap widens as the acts add up.

Even very low-risk acts add up over time. Oral sex with a partner who has HIV comes to 0.5% (about 1 in 200) over 50 acts.

What changes over time

  • Treatment. If a partner with HIV starts treatment and keeps an undetectable viral load, they stop passing HIV on through sex. Reaching undetectable can take up to six months. See U=U.
  • A new infection. If a regular partner gets HIV from someone else, their viral load is very high in the first weeks. The calculator raises the estimate 10 times for a recently infected partner.
  • Other STIs. These make HIV easier to pass on. The calculator multiplies the estimate by 2.
  • Condoms and PrEP. Condoms lower risk by about 80% in real-world use. PrEP (pre-exposure prophylaxis) taken as prescribed lowers the risk from sex by about 99% for anal sex and about 90% for vaginal sex. Ten acts of receptive anal sex on consistent PrEP with a partner who has HIV: 0.14% (about 1 in 730).

What to do now

Was the most recent exposure in the last 72 hours? PEP (post-exposure prophylaxis) can still protect you from that one, even if there were others before it. Ask at an emergency department, urgent care center or sexual health clinic, ideally within 24 hours. How to get PEP

  1. Test now. One test can cover every exposure that’s older than the window period. A lab antigen/antibody test (blood from a vein) gives a conclusive negative for anything more than 45 days ago.
  2. Retest for recent exposures. If some were inside the window, test again once it has passed. See test accuracy by week.
  3. Talk with a regular partner about testing together. Knowing both your statuses, and whether a partner with HIV is undetectable, settles most of the uncertainty.
  4. Ask about PrEP if you expect more exposures. It protects across all of them, not just one. CDC also recommends testing at least once a year if you’ve had more than one sex partner since your last test. See our HIV testing guide.