HIV spreads through specific body fluids, primarily blood, semen, rectal and vaginal fluids, and breast milk, and the risk per exposure varies enormously depending on the type of contact. Receiving a contaminated blood transfusion carries a transmission probability of roughly 93 percent per event, while a single act of vaginal intercourse carries a risk closer to one in a thousand. On the treatment side, modern antiretroviral therapy can suppress the virus to the point where it becomes undetectable in blood, which effectively eliminates the risk of passing it to a sexual partner and transforms what was once a death sentence into a manageable chronic condition. The landscape extends well beyond daily pills, though, with injectable treatments, preventive drugs for people who don’t have HIV, and experimental therapies aiming at a cure.
How the Virus Gets In
HIV targets a specific type of immune cell, the CD4+ T cell, which acts as a coordinator for much of the body’s immune response. The virus’s outer protein, known as the envelope or Env, first latches onto the CD4 receptor on the cell surface. That initial binding causes a shape change in the viral protein that exposes a second binding site, which then docks with a coreceptor, usually one called CCR5 or another called CXCR4. Only after this two-step handshake does the virus fuse with the cell membrane and inject its genetic material inside.1PubMed Central. HIV: cell binding and entry This sequence matters practically because several classes of treatment drugs are designed to interrupt it at different stages, and a rare natural mutation in the CCR5 gene makes some people highly resistant to infection.
Transmission Routes and Their Relative Risks
Not all exposures carry equal danger. A systematic review pooling data across dozens of studies estimated per-act transmission risks expressed per 10,000 exposures. Blood transfusion with contaminated blood topped the list at about 9,250 per 10,000 exposures. Mother-to-child transmission came next at roughly 2,260 per 10,000. Sexual routes were substantially lower: receptive anal intercourse carried a risk of about 138 per 10,000 exposures, receptive vaginal intercourse about 8, insertive vaginal intercourse about 4, and insertive anal intercourse about 11. Sharing needles during injection drug use came in at roughly 63 per 10,000, while an accidental needle stick in a healthcare setting was around 23 per 10,000.2PubMed Central. Estimating per-act HIV transmission risk: a systematic review
These numbers explain why blood-screening programs had such a dramatic effect on the epidemic in wealthy countries, and why anal intercourse carries higher risk than vaginal intercourse: the rectal lining is thinner and more susceptible to small tears that allow the virus direct access to underlying immune cells. The numbers also explain why a single sexual encounter, while not zero risk, is not a certainty of infection, a point that sometimes gets lost in public messaging.
HIV is not spread through saliva, sweat, tears, casual contact, sharing food, or insect bites. The virus is fragile outside the body and requires direct access to the bloodstream or to mucosal tissue to establish infection.
What Raises or Lowers the Risk of Sexual Transmission
The per-act numbers above are averages. In real life, individual risk fluctuates based on several biological cofactors. Having another sexually transmitted infection is one of the most significant. STIs like chlamydia, gonorrhea, herpes, and syphilis cause inflammation in the genital tract, which brings more immune cells, including HIV’s preferred CD4+ targets, to the mucosal surface. They can also damage the mucosal barrier, giving HIV easier access to those target cells.3PubMed Central. Mechanisms of sexually transmitted infection-induced inflammation in women: implications for HIV risk Bacterial vaginosis, which is extremely common and often has no symptoms, similarly causes genital inflammation and raises acquisition risk.4PubMed Central. Genital inflammation, immune activation and risk of sexual HIV acquisition
A study following discordant couples in Zambia, where one partner was HIV-positive and the other was not, found that genital ulceration in the HIV-positive male partner more than tripled the hazard of transmission to the female partner, while the male partner’s bilateral inguinal adenopathy, a sign of lymph node inflammation in the groin, carried a hazard ratio above 3 as well.5PubMed Central. Risk of heterosexual HIV transmission attributable to sexually transmitted infections and non-specific genital inflammation in Zambian discordant couples, 1994-2012 The practical takeaway is that treating STIs promptly and managing genital inflammation are important components of HIV prevention, not just for the STI itself but for the downstream risk of acquiring HIV.
On the other side of the equation, the most powerful factor lowering transmission risk is viral suppression in the HIV-positive partner. Condom use, circumcision in men, and pre-exposure prophylaxis for the HIV-negative partner all reduce risk as well, but viral suppression through treatment is in a category of its own.
Undetectable Equals Untransmittable
The “U=U” message, short for Undetectable equals Untransmittable, is backed by some of the most robust evidence in HIV prevention. Three landmark studies, PARTNER, PARTNER2, and Opposites Attract, followed thousands of mixed-status couples where the HIV-positive partner was on effective antiretroviral therapy with a viral load below 200 copies per milliliter. Across these studies, there were zero cases of HIV transmission linked to the treated partner, including among couples who reported having sex without condoms.6PubMed Central. The risk of sexual transmission of HIV in individuals with low-level HIV viraemia: a systematic review
The PARTNER2 study, which focused specifically on gay male couples, followed 782 couples for nearly 1,600 couple-years, encompassing more than 76,000 reports of condomless sex. No within-couple transmissions occurred. The estimated upper limit of transmission risk was one event per 435 couple-years of condomless sex, which in practical terms means effectively zero.7The Lancet. Risk of HIV transmission through condomless sex in serodifferent gay couples with the HIV-positive partner taking suppressive antiretroviral therapy (PARTNER2) This evidence holds for both anal and vaginal sex and is one of the strongest arguments for early testing and prompt treatment: a person who knows their status and takes effective therapy poses essentially no risk to their sexual partners.
Antiretroviral Therapy
Modern HIV treatment uses combinations of antiretroviral drugs that target different stages of the virus’s life cycle. These drugs are grouped into several classes. Some block the enzyme the virus uses to copy its genetic material (reverse transcriptase inhibitors). Others prevent the virus from assembling new copies of itself (protease inhibitors) or from inserting its genetic code into the host cell’s DNA (integrase inhibitors). Still others block the virus from entering cells in the first place, either by interfering with the fusion step or by blocking the CCR5 coreceptor.8PubMed Central. Mechanisms underlying of antiretroviral drugs in different cellular reservoirs with a focus on macrophages
The introduction of combination therapy in the mid-1990s was the turning point that transformed HIV from a fatal diagnosis into a manageable chronic condition. Today, most regimens involve taking a single pill once daily that contains two or three drugs from different classes. When taken consistently, these regimens suppress the virus to undetectable levels in the blood within a few months for most people.
Living on antiretroviral therapy for decades, however, comes with its own set of considerations. People on long-term treatment face a higher risk of bone loss, kidney problems, metabolic changes including cholesterol and blood sugar shifts, cardiovascular disease, and liver complications.9PubMed Central. A Review of Long-Term Toxicity of Antiretroviral Treatment Regimens and Implications for an Aging Population Some of these problems relate to the drugs themselves, while others stem from chronic low-grade inflammation that persists even when the virus is suppressed. As the HIV-positive population ages, managing these long-term effects has become a central concern in clinical care.
Pre-Exposure Prophylaxis
PrEP is a prevention strategy for people who do not have HIV but are at substantial risk of acquiring it. The most widely used oral form combines two drugs, tenofovir and emtricitabine, taken daily. When taken consistently, it reduces the risk of sexual acquisition by over 90 percent.
A newer option involves long-acting injectable cabotegravir, given as an injection every two months. The HPTN 083 trial comparing injectable cabotegravir to daily oral PrEP in men and transgender women who have sex with men found that the injectable version was substantially more effective. HIV incidence in the blinded phase was about 0.41 per 100 person-years with cabotegravir injections compared to 1.29 per 100 person-years with daily pills, translating to roughly a 69 percent lower risk with the injectable form.10The Lancet HIV. Extended evaluation of long-acting injectable cabotegravir compared with daily oral tenofovir disoproxil fumarate plus emtricitabine for pre-exposure prophylaxis in men and transgender women who have sex with men (HPTN 083) The advantage likely stems from the fact that you can’t forget to take an injection you received in a clinic, while daily pills depend on near-perfect adherence.
Lenacapavir, an even newer drug given as a subcutaneous injection just twice a year, has shown dramatic efficacy in recent trials. The move toward longer-acting prevention tools reflects a broader recognition that adherence to daily pills is the Achilles’ heel of oral PrEP, and that removing that barrier could expand protection to many more people.
Post-Exposure Prophylaxis
PEP is the emergency counterpart to PrEP. It involves taking a course of antiretroviral drugs after a potential HIV exposure, ideally within hours and no later than 72 hours. PEP is used after sexual assaults, needle-stick injuries in healthcare settings, and other high-risk incidents where someone may have been exposed. The treatment course typically lasts 28 days and uses a combination of antiretroviral drugs. The concept is straightforward: flooding the body with drugs before the virus has time to establish a permanent foothold can prevent infection from taking hold. PEP is not intended as a routine prevention strategy; it is a one-time intervention for a specific event.
Mother-to-Child Transmission
Without any intervention, an HIV-positive mother can pass the virus to her child during pregnancy, labor, delivery, or breastfeeding. The overall transmission risk per pregnancy is estimated at roughly 23 percent without treatment. Antiretroviral therapy given to the mother during pregnancy and to the infant after birth has reduced that rate to below 2 percent in well-resourced settings and remains the cornerstone of prevention.11Cochrane Database of Systematic Reviews. Interventions for reducing the risk of mother‐to‐child transmission of HIV infection
Elective cesarean delivery further reduces risk when the mother’s viral load is not fully suppressed. In cases where the mother achieves an undetectable viral load before delivery, guidelines in many countries now recommend vaginal delivery as safe, since there is little additional risk. The infant typically receives a short course of prophylactic antiretroviral medication regardless. In one documented case of a vertically infected woman with multiclass drug resistance, a tailored regimen achieved an undetectable viral load before delivery, and the newborn tested negative for HIV at every follow-up point through 48 weeks of life.12Heliyon. Preventing HIV mother-to-child transmission in a vertically infected pregnant woman with multiclass drug resistance
Breastfeeding remains a complicated area. The virus can be transmitted through breast milk, but in settings where formula feeding is unsafe due to contaminated water, the benefits of breastfeeding often outweigh the risk, particularly when the mother is on effective therapy. Achieving full elimination of mother-to-child transmission will likely require combining antiretroviral therapy with immune-based strategies that protect infants during breastfeeding.13PubMed Central. Maternal Interventions to Prevent Mother-To-Child Transmission of HIV: Moving Beyond Antiretroviral Therapy
Drug Resistance
HIV mutates rapidly, and drug-resistant variants can emerge when treatment is taken inconsistently or when drug levels in the body are too low to fully suppress viral replication. In theory, resistant strains are generated every day even in untreated people, but they normally die off because they replicate less efficiently than the dominant wild-type virus. Selective drug pressure changes that balance: a variant that can resist the drug suddenly has a survival advantage and takes over the viral population.14PubMed Central. HIV-1 Drug Resistance and Resistance Testing
Some drug classes are easier to develop resistance against than others. Older non-nucleoside reverse transcriptase inhibitors, for instance, can be rendered useless by a single mutation, while integrase inhibitors like dolutegravir have a much higher genetic barrier, meaning multiple mutations would need to accumulate simultaneously for resistance to emerge. This is one reason modern first-line regimens favor integrase inhibitors.
Resistance is not just a problem for individuals who miss doses. It can also be transmitted. A study using sensitive real-time testing found that even minority resistant variants, present at low levels before treatment starts, predicted treatment failure. Among participants with detectable minority resistance mutations at baseline, 78 percent experienced virologic failure, compared to a much lower rate in those without such mutations.15PLoS Medicine. Minority HIV-1 Drug Resistance Mutations Are Present in Antiretroviral Treatment–Naïve Populations and Associate with Reduced Treatment Efficacy This is why resistance testing before starting therapy, and again if treatment fails, is standard practice in well-resourced settings. In lower-income countries, where resistance testing infrastructure is limited, researchers have identified a panel of just six key mutations that can detect the vast majority of resistance to first-line regimens, making point-of-care testing feasible.16PLoS ONE. HIV-1 Drug Resistance Mutations: Potential Applications for Point-of-Care Genotypic Resistance Testing
Testing and Early Detection
You cannot treat what you have not diagnosed, and HIV testing has evolved substantially. Current fourth-generation tests detect both HIV antibodies and a viral protein called p24 antigen, which appears in the blood before antibodies develop. This dual approach shortens the window period, the gap between infection and a detectable test result, to about two to three weeks for most people, compared to several weeks longer with older antibody-only tests.17PubMed Central. Performance of a Fourth-Generation HIV Screening Assay and an Alternative HIV Diagnostic Testing Algorithm Rapid point-of-care tests and self-testing kits have also made it possible to get results in minutes, though these typically detect only antibodies and have a slightly longer window period.
Starting treatment early, during acute infection, has clear benefits. Acute HIV triggers an intense immune response, including a flood of inflammatory signaling molecules and shifts in immune cell populations. Starting antiretroviral therapy at the earliest stages of infection can blunt this inflammatory storm and preserve more immune function than waiting.18PubMed Central. Association between the cytokine storm, immune cell dynamics, and viral replicative capacity in hyperacute HIV infection Early treatment also shrinks the viral reservoir, the pool of quietly infected cells that makes HIV so difficult to cure, which has implications for long-term health and possibly for future cure efforts.
Immune Reconstitution Inflammatory Syndrome
There is an ironic twist to starting treatment when someone already has advanced HIV disease. Some people deteriorate shortly after beginning therapy despite the drugs working exactly as intended. This reaction, called immune reconstitution inflammatory syndrome or IRIS, happens as the immune system rebounds. Newly recovered CD4+ T cells encounter infections that had been quietly lingering, like tuberculosis, fungal infections, or certain viral infections, and mount an aggressive inflammatory response that can cause serious tissue damage.19PubMed Central. Immune reconstitution inflammatory syndrome: the trouble with immunity when you had none IRIS is most common in people who start therapy with very low CD4 counts and underscores why early diagnosis and treatment, before the immune system is severely depleted, produces far better outcomes than waiting.
Long-Acting Injectables and the Future of Treatment
Daily pills work, but they require lifelong adherence, and even small lapses open the door to resistance and viral rebound. Long-acting injectable formulations aim to solve this. The combination of cabotegravir and rilpivirine, given as two injections every two months, is already approved for treatment in people who are already virally suppressed. Looking further ahead, modeling research has evaluated an injectable combination of lenacapavir and cabotegravir for treatment settings in Africa and found that it could substantially reduce HIV deaths and disability, including by lowering mother-to-child transmission rates, if the drugs can be sourced at around $80 per person per year or less.20Nature Communications. Potential impact and cost-effectiveness of long-acting injectable lenacapavir plus cabotegravir as HIV treatment in Africa
Beyond conventional antivirals, broadly neutralizing antibodies represent a fundamentally different therapeutic approach. These are lab-engineered versions of the antibodies some people naturally produce that can neutralize a wide range of HIV strains by targeting conserved structures on the viral surface. Clinical trials have shown that combinations of at least two broadly neutralizing antibodies can maintain viral suppression after standard therapy is stopped, though this works only when the person’s virus is sensitive to the particular antibodies used.21PubMed Central. Future of bNAbs in HIV Treatment Single-antibody infusions produced only temporary viral load drops before resistant variants emerged, which is why combination approaches are essential.22PubMed Central. Broadly neutralizing antibodies for the treatment and prevention of HIV infection The goal is to pair these antibodies with injectable antiretrovirals to create regimens requiring as few as two clinic visits per year, and researchers are also combining them with immune modulators in hopes of achieving sustained remission without any ongoing therapy.23PubMed. Broadly neutralizing antibodies for HIV treatment and cure approaches
The Reservoir Problem and Why a Cure Remains Elusive
Antiretroviral therapy suppresses HIV but does not eliminate it. The virus persists in a latent form within resting memory CD4+ cells and other long-lived immune cells that carry an integrated copy of the viral genome. These cells can survive for decades, and because the virus is not actively replicating inside them, drugs that target viral replication cannot reach it. If therapy is stopped, the virus can reactivate from these reservoirs and rebound within weeks.24PubMed Central. Stem cell transplantation in strategies for curing HIV/AIDS
Stem cell transplantation has produced a handful of apparent cures, most famously in patients who received bone marrow from donors carrying the CCR5 mutation that blocks HIV entry. But these transplants were performed to treat blood cancers, not HIV itself. The procedure carries serious risks, including graft-versus-host disease and death, and requires a matched donor with the rare mutation. It is not scalable as a general treatment strategy. Research is now focused on gene-editing techniques that could knock out CCR5 in a patient’s own cells, as well as “shock and kill” strategies designed to wake up latent virus so the immune system or drugs can destroy the cells harboring it. None of these approaches has yet produced a reliable, broadly applicable cure.
Elite Controllers and Natural Resistance
A small fraction of people infected with HIV, often estimated at under 1 percent, spontaneously suppress the virus to undetectable levels without any treatment. These “elite controllers” have been intensely studied for clues about what a functional cure might look like. Research points to exceptionally potent CD8+ T cells, the immune system’s killer cells, as the main driver of this control, supported by finely coordinated innate immune responses. Interestingly, elite controllers also appear to harbor their intact viral DNA in parts of the genome where it is less likely to be activated, creating a state of particularly deep latency. Understanding how this natural control works could eventually inform vaccine design or therapeutic strategies for the broader HIV-positive population.
Where HIV Came From
HIV did not emerge from nowhere. Both HIV-1 and HIV-2, the two types that infect humans, originated from simian immunodeficiency viruses circulating naturally in African primates.25PubMed Central. Origins of HIV and the AIDS pandemic HIV-1, which causes the vast majority of infections worldwide, traces back to chimpanzees and possibly gorillas, with phylogenetic analysis revealing at least four separate cross-species transmission events.26PubMed Central. The evolution of HIV-1 and the origin of AIDS HIV-2, which is less transmissible and largely confined to West Africa, came from a different primate species, the sooty mangabey. These zoonotic jumps are thought to have occurred through bushmeat hunting and butchering, which exposed people to infected primate blood. The virus then adapted to human-to-human transmission and spread through populations over decades before it was recognized as a new disease in the early 1980s. Understanding this zoonotic origin has been important for tracing the virus’s evolution and its remarkable genetic diversity, which is one of the main reasons developing a universal vaccine has proved so difficult.