HIV spreads through specific body fluids: blood, semen (including pre-seminal fluid), vaginal fluids, rectal fluids, and breast milk. For transmission to occur, one of these fluids from a person with HIV must enter another person’s body through a mucous membrane, broken skin, or direct injection into the bloodstream. The most common routes are sex without a condom, shared injection equipment, and from parent to child during pregnancy or birth.
Sexual Transmission
Sex is the most common way HIV spreads worldwide, but the risk varies dramatically depending on the type of sex. Mucous membranes line the rectum, vagina, the opening of the penis, and the mouth, and these thin tissues allow HIV to enter the body. The virus can also get in through cuts, sores, or the foreskin.
Per-act risk estimates from the CDC, assuming no condoms, no PrEP, and no treatment for the partner with HIV, look like this:
- Receptive anal sex: roughly 1 in 72 (the highest-risk sexual act)
- Insertive anal sex: roughly 1 in 909
- Receptive vaginal sex: roughly 1 in 1,250
- Insertive vaginal sex: roughly 1 in 2,500
Oral sex carries extremely low to negligible risk. Kissing and touching do not transmit HIV.
These numbers represent averages across many exposures. In real life, individual risk can be higher or lower depending on factors like viral load, whether other sexually transmitted infections are present, and whether there are open sores or inflammation. A single encounter can result in transmission, even if the statistical average seems low.
How Other STIs Raise the Risk
Having another sexually transmitted infection significantly increases the chance of both acquiring and passing on HIV. Gonorrhea carries the strongest association, conferring roughly a 7-fold increase in HIV risk. Recurrent rectal gonorrhea or chlamydia infections raise it more than 8-fold. Genital ulcers, including those from herpes, increase per-act infectivity about 5-fold. Even high-risk HPV is associated with a 4-fold increase.
This works in both directions. STIs cause inflammation and sometimes open sores in the genital or rectal lining, making it easier for HIV to enter. In someone already living with HIV, STIs like urethritis and cervicitis increase the amount of virus present in genital fluids, making them more infectious. Treating and preventing other STIs is one of the most underappreciated ways to reduce HIV transmission.
Shared Needles and Injection Equipment
Sharing needles, syringes, or other injection equipment is a highly efficient route of transmission because infected blood is delivered directly into the bloodstream. The estimated risk per injection with a contaminated syringe is about 0.67%, or roughly 1 in 150. That’s several times higher than the per-act risk of vaginal sex.
HIV can survive inside a used syringe far longer than on an open surface. At room temperature, viable virus has been recovered from syringes for up to 21 days. At cooler temperatures (around 4°C), half of tested syringes still contained live virus at 42 days. At warmer temperatures above 27°C, the likelihood of viable virus drops to less than 1% after one week. This means syringes left in cooler environments, like those stored or stashed for later use, pose a real risk even days or weeks after first use.
From Parent to Child
HIV can pass from a parent to a child during pregnancy, labor, delivery, or breastfeeding. Without any treatment, the overall transmission rate ranges from 15% to 45%. Most of that risk, about 65%, is concentrated around the time of labor and delivery. Breastfeeding adds an additional 7% to 22% risk if the parent is not on treatment.
With proper treatment during pregnancy and a managed delivery plan, that rate drops to below 2%. This is one of the great success stories in HIV prevention. Routine prenatal screening and early treatment have made parent-to-child transmission rare in countries with good access to care.
How HIV Does Not Spread
HIV does not survive well outside the body on surfaces, in water, or in the air. You cannot get HIV from shaking hands, hugging, sharing food or drinks, using the same toilet, swimming in the same pool, or being bitten by mosquitoes. Saliva, sweat, tears, and urine do not carry enough virus to cause infection. Even in scenarios involving small amounts of blood, like a scraped knee at a playground, transmission would require an unusually large amount of blood and a direct entry point such as an open wound.
Undetectable Means Untransmittable
One of the most important developments in HIV science is the concept known as U=U: undetectable equals untransmittable. A person living with HIV who takes antiretroviral therapy and maintains an undetectable viral load has zero risk of transmitting the virus to sexual partners. This isn’t a rough estimate or a theoretical model. Large studies tracking thousands of couples where one partner had HIV and one did not recorded zero transmissions when the partner with HIV maintained an undetectable viral load.
This applies specifically to sexual transmission. Treatment as prevention is considered one of the most effective biomedical tools available for stopping the spread of HIV.
PrEP and PEP: Prevention After Exposure or Before
PrEP is a daily medication taken by people who do not have HIV but are at higher risk of exposure. When taken as prescribed, PrEP reduces the risk of getting HIV from sex by about 99%. For people who inject drugs, it reduces risk by at least 74%.
PEP is an emergency option for people who may have already been exposed. It involves a course of medication that should ideally be started within 24 hours of exposure, and no later than 72 hours (3 days). After that window, it won’t work. PEP is effective but not 100% guaranteed, so it’s a backup measure rather than a primary prevention strategy.
What Actually Determines Your Risk
The factors that matter most in any potential exposure are the type of contact, the viral load of the person with HIV, the presence of other STIs, and whether any prevention tools are in play. A person on effective treatment with an undetectable viral load poses no sexual transmission risk. Condoms, when used consistently, reduce risk dramatically. PrEP adds another layer of near-complete protection. These tools can be combined, and in practice, most new infections occur in situations where none of them are being used.