HIV spreads through five 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, typically through a mucous membrane, an open wound, or a direct injection into the bloodstream. The vast majority of new infections happen through sex without a condom or through sharing needles.
Sexual Transmission
Unprotected vaginal and anal sex are the most common ways HIV spreads worldwide. During sex, the virus in semen, vaginal fluid, or rectal fluid can enter the body through the thin, absorbent tissue lining the genitals, rectum, or, less commonly, the mouth. Once inside, HIV targets a type of immune cell and begins replicating, eventually establishing a permanent infection.
Not all sexual acts carry the same level of risk. The CDC estimates the per-act chance of getting HIV from an untreated partner (without condoms or preventive medication) as follows:
- Receptive anal sex: roughly 1 in 72 (the highest-risk sexual activity)
- Insertive anal sex: roughly 1 in 909
- Receptive vaginal sex: roughly 1 in 1,250
- Insertive vaginal sex: roughly 1 in 2,500
Receptive anal sex is by far the riskiest because the rectal lining is thin, has a rich blood supply, and is more prone to small tears during sex. Vaginal tissue is thicker and more resilient, which partly explains the lower per-act numbers. These are averages across many encounters. Having another sexually transmitted infection, genital sores, or a higher viral load in the HIV-positive partner can raise the risk substantially in any single encounter.
Sharing Needles and Drug Equipment
When someone injects drugs with a needle that was used by a person with HIV, blood containing the virus can be pushed directly into their bloodstream. The estimated risk per injection with a contaminated syringe is about 0.67%, or roughly 1 in 150. That may sound low for a single event, but people who share equipment often do so repeatedly, and the cumulative risk adds up fast.
It’s not just the needle itself. Cookers, cotton filters, and rinse water can also carry enough blood to transmit the virus. Needles used for tattooing or piercing pose a risk too if they aren’t properly sterilized, though this is uncommon in licensed settings.
From Parent to Child
HIV can pass from mother to baby during pregnancy, labor and delivery, or breastfeeding. Without any treatment, the transmission rate is significant, though fewer than half of exposed infants become infected. With antiretroviral treatment during and after pregnancy, the risk drops to under 5% in real-world conditions, and in well-resourced healthcare systems it falls below 1%.
The virus can cross the placenta during pregnancy, enter the baby’s bloodstream during the physical process of birth, or pass through breast milk over months of nursing. This is why HIV testing during pregnancy is now standard in most countries. When a pregnant person is diagnosed and begins treatment early, the chances of their baby being born HIV-free are very high.
Blood Transfusions and Healthcare Settings
Before routine blood screening began in the mid-1980s, contaminated blood transfusions were a significant source of HIV transmission. Today, donated blood is tested for HIV in virtually every country, making this route extremely rare in modern healthcare systems.
Healthcare workers face a small occupational risk from accidental needle sticks. The chance of acquiring HIV from a single needle stick involving an infected patient’s blood is about 0.3%, or 3 in 1,000. Strict protocols for handling sharp instruments, along with the availability of post-exposure medication, have made workplace transmission uncommon.
How HIV Does Not Spread
HIV is not transmitted through saliva, sweat, tears, or urine. You cannot get HIV from kissing (including deep kissing), sharing food or drinks, hugging, shaking hands, or using the same toilet. Mosquitoes and other insects do not spread it. The virus is fragile outside the body and cannot survive on surfaces for any meaningful amount of time.
In extremely rare cases, HIV has been linked to sharing razors or toothbrushes with someone who is HIV-positive, but only when visible blood was present and the other person had an open cut or sore. Casual, everyday contact with someone living with HIV carries zero risk.
What “Undetectable” Means for Transmission
One of the most important developments in HIV science is the confirmation that people on effective treatment who maintain an undetectable viral load have 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’s virus was fully suppressed by medication.
An undetectable viral load means the amount of virus in the blood is too low to be picked up by standard tests. This typically requires taking antiretroviral medication consistently. The result is that treatment doubles as prevention, both protecting the person’s own health and eliminating the chance of passing the virus to others through sex.
How Prevention Reduces Risk
Condoms remain one of the most effective and accessible tools for preventing sexual transmission of HIV. When used consistently and correctly, they block the exchange of the fluids that carry the virus.
Pre-exposure prophylaxis (PrEP) is a daily or on-demand medication for people who are HIV-negative but at higher risk. When taken as recommended, PrEP reduces the risk of getting HIV from sex by about 99% and from injection drug use by at least 74%. For people who share injection equipment, needle exchange programs and access to clean syringes dramatically lower the odds of transmission as well.
The combination of condoms, PrEP, treatment as prevention (U=U), and clean needle access means that virtually every route of HIV transmission now has an effective countermeasure. The virus spreads through specific, well-understood pathways, and each of those pathways can be interrupted.