HIV spreads through specific body fluids: blood, semen, pre-seminal fluid, rectal fluids, vaginal fluids, and breast milk. For transmission to happen, one of these fluids from a person with a detectable viral load must contact a mucous membrane, damaged tissue, or enter the bloodstream directly through a needle. Most people get HIV through anal or vaginal sex or sharing injection drug equipment.
Sexual Transmission
Sex is the most common way HIV spreads. The virus passes through mucous membranes found inside the rectum, vagina, penis (at the urethra opening), and mouth. Either partner can get HIV during anal or vaginal sex, though the risk varies significantly depending on the act.
Receptive anal sex carries the highest per-act risk of any sexual activity: roughly 138 per 10,000 exposures without any protection. That’s because the rectal lining is thin and easily damaged, giving the virus direct access. Insertive anal sex is lower at about 11 per 10,000 acts. For vaginal sex, the receptive partner faces a risk of about 8 per 10,000 acts, while the insertive partner’s risk is around 4 per 10,000. These numbers reflect unprotected sex with no medication involved.
Oral sex carries extremely low to negligible risk. While mucous membranes are present in the mouth, the combination of saliva and the thickness of oral tissue makes this route very unlikely.
Several factors can raise the risk during any sexual encounter. Having other sexually transmitted infections, open sores, or cuts creates additional entry points for the virus. A higher viral load in the HIV-positive partner also increases the chance of transmission.
Sharing Needles and Injection Equipment
Sharing needles, syringes, or other injection equipment is the second most common route of transmission. When someone injects drugs, small amounts of blood remain inside the needle and syringe. If another person uses that same equipment, the blood (and the virus it carries) gets injected directly into their bloodstream, bypassing the body’s outer defenses entirely.
This applies to all shared injection equipment, not just the needle itself. Cookers, cotton filters, and rinse water can also harbor blood. The risk isn’t limited to recreational drug use; any situation where needles are reused without proper sterilization poses a potential concern.
From Parent to Child
HIV can pass from mother to baby during pregnancy, labor, delivery, or breastfeeding. Without any treatment, the transmission rate ranges from 15% to 45%. That’s a substantial risk, but it’s also one of the most preventable routes. With antiretroviral treatment during pregnancy and after birth, the risk drops so dramatically that virtual elimination of parent-to-child transmission has been achieved in some settings.
Which Body Fluids Don’t Transmit HIV
A lot of fear around HIV comes from misunderstanding which fluids are actually infectious. Saliva, tears, sweat, urine, and feces (when not mixed with blood) do not carry enough virus to transmit HIV. There are no documented cases of HIV being transmitted through spitting.
HIV also does not spread through:
- Air or water
- Mosquitoes, ticks, or other insects
- Casual contact like hugging, shaking hands, or sharing food
- Toilet seats or swimming pools
The virus is fragile outside the body. It requires very specific conditions to survive and cannot replicate without living human cells, which is why environmental exposure (touching surfaces, for example) is not a realistic route of infection.
Why Viral Load Matters
Not everyone living with HIV can transmit the virus. A person on effective treatment who maintains an undetectable viral load has zero risk of transmitting HIV to sexual partners. This principle, known as Undetectable = Untransmittable (U=U), is backed by large studies involving thousands of couples where no transmissions occurred when the HIV-positive partner was virally suppressed.
This is a critical point: HIV transmission requires a detectable viral load. Someone who takes their medication consistently and has achieved undetectable status cannot pass the virus through sex.
How Prevention Reduces Risk
Two medical tools significantly reduce the chance of getting HIV, even during high-risk exposures.
PrEP (pre-exposure prophylaxis) is medication taken by HIV-negative people before potential exposure. When taken as prescribed, PrEP reduces the risk of getting HIV from sex by about 99%. For people who inject drugs, PrEP pills reduce risk by at least 74%.
PEP (post-exposure prophylaxis) is an emergency option taken after a potential exposure. It must be started within 72 hours and involves a course of antiretroviral medication lasting 28 days.
Condoms also remain effective at blocking the body fluids responsible for transmission. When used consistently, they substantially lower the risk during both anal and vaginal sex. Combining condoms with PrEP provides the highest level of protection available outside of abstinence.