Making sense of HIV risk

Two things have to be true

For an exposure to pass HIV on, the other person has to have HIV, and the virus has to get from them into your bloodstream. Each of those is a probability, and your overall risk is roughly one multiplied by the other. That's why the same act can carry very different risk depending on who it was with.

If someone's HIV status is unknown, the first probability is usually small. In much of Western Europe it's around 1 in 500 adults. Even in the countries with the highest prevalence, most people do not have HIV.

Risk is counted one act at a time

The figures you'll see, such as about 1.4% for receptive anal sex with a partner who has HIV, are averages per act. They come from studies that followed many couples over time. They mean that, on average, transmission happens in roughly 1 of every 70 such acts. That isn't a countdown: it could happen the first time or not at all over many encounters.

Repeated exposures do add up. Ten acts at 1% each don't make 10%; the combined chance is a little under that (about 9.6%), because each act is its own chance. The calculator does this sum for you.

What makes risk higher

• The type of exposure. Receptive anal sex carries the highest sexual risk because the lining of the rectum is thin and easily damaged. Sharing injection equipment puts blood directly into the bloodstream.
• A high viral load. The amount of virus in someone's blood is the biggest driver. It peaks in the first weeks after infection, often before the person knows they have HIV.
• Other STIs. Infections like syphilis, gonorrhea and herpes cause inflammation and sores that make it easier for HIV to enter.
• Repetition. More exposures mean more chances.

What makes risk lower, sometimes to zero

• Treatment. A person with HIV who keeps an undetectable viral load on treatment does not pass HIV on through sex. This is the basis of U=U (undetectable = untransmittable).
• PrEP. Taken as prescribed, PrEP lowers the risk of getting HIV from sex by about 99%.
• PEP. Started within 72 hours of an exposure, PEP can stop an infection from taking hold.
• Condoms. Used correctly every time, condoms are highly effective. Real-world estimates of about 80% protection include occasional slips and breaks.
• Circumcision. Lowers risk for the insertive partner by roughly half in vaginal sex.

Contact that carries no risk

HIV does not survive well outside the body and isn't spread by everyday contact. You can't get HIV from:

• Kissing, hugging, or shaking hands
• Saliva, sweat, or tears
• Sharing cups, cutlery, food, toilets, or swimming pools
• Mosquitoes or other insects
• Touching unbroken skin, even with blood on it

What to do with your number

Use it to decide how urgently to act, not to diagnose yourself. A high estimate inside 72 hours means getting PEP now. A low estimate is reassuring, but a test is still the only way to know for sure, and the calculator will show you when each type of test can give a reliable answer.