HIV spreads through specific body fluids: blood, semen, pre-seminal fluid, rectal fluids, vaginal fluids, and breast milk. These fluids must contact a mucous membrane, damaged tissue, or enter the bloodstream directly for transmission to occur. The vast majority of new infections come from anal or vaginal sex without protection, sharing injection drug equipment, or transmission from parent to child during pregnancy, birth, or breastfeeding.
How HIV Is Transmitted During Sex
Not all sexual exposure carries the same level of risk. Receptive anal sex (being the bottom partner) is the highest-risk sexual activity, with roughly a 1 in 72 chance of transmission per act when no protection is used and the other partner has HIV. The lining of the rectum is thin and rich in immune cells that HIV targets, making it a particularly vulnerable entry point.
Insertive anal sex carries a much lower per-act risk of about 1 in 909. For vaginal sex, the receptive partner faces roughly a 1 in 1,250 chance per act, while the insertive partner’s risk is about 1 in 2,500. These numbers assume no condoms, no preventive medication, and an HIV-positive partner who isn’t on treatment. In real-world conditions, factors like other sexually transmitted infections, genital sores, or higher viral loads can push the risk higher.
Oral sex carries a very low but not zero risk. HIV can enter through mucous membranes in the mouth, especially if there are cuts, sores, or gum disease present.
Transmission Through Needles and From Parent to Child
Sharing needles, syringes, or other injection equipment is a major route of transmission. Used equipment can retain blood inside it, and injecting that blood directly into the body is an efficient way for the virus to spread. This includes not just the needle itself but also cookers, cotton filters, and rinse water used to prepare drugs.
HIV can also pass from a pregnant or breastfeeding parent to their baby. Without treatment, the risk of perinatal transmission is significant. With proper antiretroviral treatment during pregnancy, delivery, and breastfeeding, that risk drops to below 1%.
How HIV Does Not Spread
HIV does not survive well outside the human body. You cannot get HIV from casual contact like hugging, shaking hands, sharing food or drinks, or using the same toilet. Saliva, sweat, and tears do not carry enough virus to transmit infection. Mosquitoes and other insects do not spread HIV either.
How Antiretroviral Treatment Works
HIV treatment uses antiretroviral therapy (ART), a combination of medications that block the virus at different stages of its life cycle. Some drugs prevent the virus from copying its genetic material inside your cells. Others stop it from assembling new viral particles or from entering cells in the first place. By attacking the virus from multiple angles simultaneously, ART keeps it from multiplying and allows your immune system to recover.
Current global guidelines recommend starting treatment immediately after diagnosis, regardless of how healthy you feel or what your immune cell counts look like. The preferred first-line regimens are built around a class of drugs that block the virus from inserting its genetic code into your cells’ DNA. These regimens are highly effective, well-tolerated, and typically involve taking one or two pills per day. If that first approach doesn’t work or causes side effects, alternative regimens using different drug classes are available.
The goal of treatment is to reduce the amount of virus in your blood (your viral load) to undetectable levels, defined as fewer than 200 copies per milliliter of blood. Most people who start treatment and take it consistently reach undetectable status within a few months.
What Undetectable Means for Transmission
This is one of the most important developments in HIV science: a person living with HIV who maintains an undetectable viral load cannot transmit the virus to sexual partners. This principle, known as U=U (undetectable equals untransmittable), is backed by large studies involving thousands of couples where zero transmissions occurred when the HIV-positive partner was virally suppressed. Treatment isn’t just about staying healthy. It completely eliminates the risk of passing HIV through sex.
Long-Acting Injectable Treatment
For people who find it difficult to take daily pills or simply prefer not to, long-acting injectable treatment is now available. A two-drug injectable regimen can be given once a month or once every two months, replacing daily oral medication entirely. To qualify, you need to already have an undetectable viral load on a stable oral regimen, with no history of treatment failure or drug resistance. After an initial phase of monthly injections, many people switch to the every-two-month schedule. Each visit involves two intramuscular injections, typically given at a clinic.
Preventing HIV Before and After Exposure
Pre-exposure prophylaxis (PrEP) is a medication taken by HIV-negative people to prevent infection. 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%. PrEP is available as a daily pill or as an injectable given every two months.
Post-exposure prophylaxis (PEP) is an emergency option for people who may have been exposed to HIV. PEP must be started within 72 hours of exposure, and the sooner the better. It involves taking antiretroviral medications every day for 28 days. PEP is not intended as a regular prevention strategy but as a backup after a condom breaks, after a sexual assault, or after an accidental needle stick.
Getting Tested and Window Periods
No HIV test can detect the virus immediately after exposure. Each type of test has a window period, the time between potential exposure and when the test can reliably detect infection:
- Nucleic acid tests (NAT): detect HIV 10 to 33 days after exposure. These look for the virus itself in your blood and are the earliest to turn positive.
- Lab-based antigen/antibody tests (using blood drawn from a vein): 18 to 45 days after exposure.
- Rapid antigen/antibody tests (finger-stick blood): 18 to 90 days after exposure.
- Antibody-only tests: 23 to 90 days after exposure.
If you test negative but your potential exposure was recent, you may need to test again after the window period has fully passed. A negative result on a lab-based antigen/antibody test taken 45 or more days after exposure is highly reliable for most people.
Living With HIV Today
HIV is no longer a fatal diagnosis. With consistent treatment, people living with HIV have a near-normal life expectancy. The virus can be suppressed to undetectable levels, protecting both your health and your partners. Treatment regimens have become simpler, with fewer side effects than earlier generations of drugs, and options now exist for people who prefer injections over daily pills. The combination of effective treatment, PrEP for prevention, and the reality of U=U has fundamentally changed what it means to live with or be at risk for HIV.