HIV (the virus that causes AIDS) spreads through specific bodily fluids: blood, semen, pre-seminal fluid, rectal fluids, vaginal fluids, and breast milk. These fluids must enter the bloodstream through a mucous membrane, broken skin, or direct injection for transmission to occur. Outside of these pathways, HIV does not spread through everyday contact.
Sexual Contact
Sex without a condom is the most common way HIV is transmitted worldwide. During intercourse, the virus can pass through the thin mucous membranes lining the rectum, vagina, urethra, and mouth. Even without visible cuts or tears, tiny abrasions that occur naturally during sex create entry points. Once past this barrier, immune cells in the underlying tissue pick up viral particles and carry them deeper into the body, where the infection takes hold.
Not all sexual acts carry equal risk. Per-act transmission estimates from the CDC (assuming no condoms, no preventive medication, and no treatment) look like this:
- Receptive anal sex: roughly 1 in 72 chance per act
- 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 carries the highest risk because the rectal lining is thinner and more prone to micro-tears than vaginal tissue. The receptive partner in any act faces greater risk than the insertive partner, though transmission can go in either direction. Oral sex carries a much lower risk, though it is not zero if there are open sores, bleeding gums, or contact with semen or vaginal fluid.
Sharing Needles and Syringes
When someone injects drugs with a needle that was used by a person living with HIV, blood containing the virus can be pushed directly into the bloodstream. This is one of the most efficient routes of transmission because it bypasses every natural barrier the body has. Sharing any injection equipment, not just the needle itself, increases risk. Cookers, cotton filters, and rinse water can all hold enough residual blood to transmit the virus.
Needlestick injuries in healthcare settings are a separate but real concern, though the average risk from a single accidental stick with an HIV-contaminated needle is low (about 0.23%). Post-exposure preventive treatment, started within 72 hours, reduces that risk further.
From Parent to Child
HIV can pass from a mother to her baby during pregnancy, labor and delivery, or breastfeeding. Without any treatment, the transmission rate during pregnancy and birth can be as high as 15 to 45%. With proper treatment throughout pregnancy and delivery, that number drops dramatically. For mothers who maintain an undetectable viral load on treatment, the risk of transmitting HIV through breastfeeding is less than 1%, though not zero. Using properly prepared formula or pasteurized donor milk eliminates the breastfeeding risk entirely.
Blood Transfusions and Organ Transplants
In the early years of the AIDS epidemic, contaminated blood products were a significant source of infection. That is no longer the case in countries with modern screening. In the United States, every blood donation is tested for HIV using both antibody detection and nucleic acid testing, which can identify the virus even in very recent infections. The current risk of receiving HIV-contaminated blood from a screened donation is estimated at less than 1 in a million. Organ and tissue donors undergo similar screening.
What Does Not Transmit HIV
HIV cannot survive or replicate outside the human body for long. It is not airborne and does not spread through water, food, or insect bites. The following do not transmit HIV:
- Casual contact: hugging, shaking hands, sharing a toilet seat, touching doorknobs
- Saliva: kissing (closed mouth), sharing utensils, coughing, sneezing
- Other body fluids: sweat, tears, urine, mucus
There is a narrow, extremely rare exception: if a significant amount of blood mixes with saliva (from bleeding gums, open sores, or an injury), a theoretical risk exists, though documented cases are virtually nonexistent. In practical terms, saliva alone does not carry enough virus to cause infection, and enzymes in saliva further inhibit HIV.
How Treatment Prevents Transmission
One of the most important facts about HIV transmission today is the concept known as U=U: undetectable equals untransmittable. A person living with HIV who takes antiretroviral treatment and maintains an undetectable viral load has zero risk of transmitting the virus to sexual partners. This has been confirmed in large studies tracking thousands of couples over years, with no linked transmissions when the HIV-positive partner was virally suppressed. Treatment doesn’t just protect the person taking it; it stops the chain of transmission entirely.
How Prevention Works for HIV-Negative People
PrEP (pre-exposure prophylaxis) is a medication that HIV-negative people can take to prevent infection. 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%. PrEP is available as a daily pill or a long-acting injection given every two months, making it a practical option for people at ongoing risk.
Condoms remain highly effective at blocking HIV during sex. When used consistently and correctly, latex and polyurethane condoms prevent contact between infectious fluids and mucous membranes. Combining condoms with PrEP or a partner’s viral suppression through treatment layers protection even further, though any one of these methods on its own is already highly effective.