Sexually transmitted infections spread primarily through vaginal, anal, and oral sex, but those are not the only routes. Blood transfusions, shared needles, mother-to-child transmission during birth or breastfeeding, organ transplants, and even shared sex toys can all carry infections from one person to another. The risk per encounter varies enormously depending on the pathogen, the type of contact, and the biological circumstances of both people involved. Understanding both the sexual and the less obvious non-sexual pathways clears up persistent myths and has real practical implications for prevention.
Not All Sexual Acts Carry the Same Risk
One of the most important things to understand about STI transmission is that the type of sexual contact matters a great deal. A systematic review estimating per-act HIV transmission risk found that receptive anal intercourse carried the highest sexual risk at roughly 138 infections per 10,000 exposures, while receptive vaginal intercourse came in around 8 per 10,000, insertive vaginal intercourse around 4 per 10,000, and oral sex was very low, with the confidence interval crossing zero.1PubMed Central. Estimating per-act HIV transmission risk: a systematic review Those numbers are specific to HIV during its chronic phase. For other common STIs like gonorrhea, chlamydia, syphilis, and trichomoniasis, the picture looks very different: per-act transmission probabilities can be 20 to 50 percent, making them far easier to catch from a single encounter.2PubMed Central. Linearity and Nonlinearity in HIV/STI Transmission: Implications for the Evaluation of Sexual Risk Reduction Interventions
The difference is staggering. A single unprotected vaginal encounter with an HIV-positive partner during chronic infection carries a risk well under 1 percent for the receptive partner. A single encounter with someone who has gonorrhea could carry a risk above 20 percent. This is why STIs like chlamydia and gonorrhea remain so common globally despite widespread condom availability: even modest gaps in protection translate into a lot of infections.
Anal tissue is thinner and more vulnerable to micro-tears than vaginal tissue, which helps explain the higher transmission rates for receptive anal intercourse. But anatomy alone does not determine everything. The stage of infection in the source partner also plays a role. Research on HIV viral loads in semen found that concentrations are much higher during the acute phase (the first weeks after infection) and again in late-stage disease, providing a biological explanation for the bursts of transmission that occur during those windows.3PubMed Central. Amplified transmission of HIV-1: comparison of HIV-1 concentrations in semen and blood during acute and chronic infection This is part of why someone recently infected who does not yet know their status can be especially likely to pass the virus along.
The Asymptomatic Problem
A recurring theme in STI epidemiology is that many infections spread most efficiently when the carrier has no symptoms. Herpes simplex virus is the classic example. Studies of oral HSV shedding in people without visible sores found that HSV-1 DNA was present in the saliva of over half of seropositive individuals across multiple visits, with the virus detectable on about a third of days tested. Shedding episodes typically lasted one to three days, but in roughly 10 percent of people, episodes stretched longer.4PubMed. Asymptomatic shedding of herpes simplex virus (HSV) in the oral cavity This means someone with oral herpes can pass the virus through kissing or oral sex on days when they feel perfectly fine and have no cold sores.
Gonorrhea presents a similar challenge, especially in men who have sex with men. Research has identified the pharynx (the back of the throat) as a principal driver of gonorrhea prevalence in this population, largely because pharyngeal gonorrhea is almost always asymptomatic.5PubMed Central. Frequent Transmission of Gonorrhea in Men Who Have Sex with Men A person carrying gonorrhea in their throat may have no sore throat, no redness, and no reason to suspect anything is wrong. They can transmit the bacteria through oral sex or even through kissing, and without targeted screening, the infection circulates silently. The practical takeaway is that screening based on symptoms alone misses a large share of active infections. Routine screening of asymptomatic sites, particularly the throat and rectum, is a critical tool for bringing transmission rates down.
Mother-to-Child Transmission
Many sexually transmitted infections can cross from parent to child during pregnancy, labor, or breastfeeding. HIV is the most widely discussed: the estimated per-exposure risk for mother-to-child transmission sits around 2,260 per 10,000 exposures without medical intervention, a rate second only to contaminated blood transfusions.1PubMed Central. Estimating per-act HIV transmission risk: a systematic review Antiretroviral therapy during pregnancy and delivery has reduced this dramatically in countries where treatment is accessible, turning what was once a near-certainty in untreated cases into an uncommon event.
Syphilis, hepatitis B, herpes simplex, gonorrhea, and chlamydia can all transmit vertically as well. Congenital syphilis, in particular, has been rising in several high-income countries in recent years, a reminder that vertical transmission is not purely a concern in resource-limited settings. Herpes transmission during vaginal delivery is especially dangerous to newborns, whose immature immune systems handle the virus poorly. This is part of why cesarean delivery is recommended when a mother has active genital herpes lesions at the time of labor.
An emerging concern involves HTLV-1, a virus that can cause leukemia and is transmitted through breastfeeding. As guidelines in high-income countries have moved toward supporting breastfeeding among HIV-positive mothers under antiretroviral treatment, researchers have flagged the risk that mothers coinfected with HTLV-1 could unknowingly pass this second virus to their infants, since few countries screen for HTLV-1 during prenatal care.6PubMed Central. Emerging Risk for Human T-Cell Leukemia Virus Type 1 Transmission with HIV-Positive Breastfeeding Support
Blood, Needles, and Organ Transplants
Blood transfusion with contaminated products carries the highest per-exposure transmission risk of any route studied for HIV: roughly 9,250 infections per 10,000 exposures, or over 92 percent.1PubMed Central. Estimating per-act HIV transmission risk: a systematic review Modern blood screening in high-income countries has made this route vanishingly rare, but it remains a concern in regions where screening infrastructure is less developed.
Needle sharing among people who inject drugs carries a per-exposure risk estimated at around 63 per 10,000 for HIV. Occupational needle-stick injuries in healthcare settings are lower, at roughly 23 per 10,000. For hepatitis C, needle sharing is actually the dominant route of transmission globally, and hepatitis B can survive on surfaces for days, making accidental needle exposure a real occupational hazard.
Organ and tissue transplantation is another non-sexual route. Documented cases include HIV, hepatitis B, tuberculosis, herpes simplex, and cytomegalovirus transmitted through bone grafts, corneal transplants, heart valves, and other donor tissues.7Cell Transplantation. Infectious Disease Transmission through Cell, Tissue, and Organ Transplantation: Reducing the Risk through Donor Selection Donor screening protocols exist to minimize these risks, but no screening test is perfect, especially during the window period after infection when a donor may test negative despite being infectious.
Fomites, Surfaces, and Shared Objects
Most STIs are fragile organisms that die quickly outside the human body, which is why casual contact with surfaces like toilet seats, doorknobs, or swimming pools is not a meaningful transmission route for the vast majority of infections. But there are exceptions, and they matter.
HPV, the virus responsible for genital warts and several cancers, is unusually hardy. A study examining vaginally inserted sex toys found that HPV was detectable on vibrators immediately after use in women with vaginal HPV, and the virus persisted up to 24 hours after standard cleaning.8Sexually Transmitted Infections. A study of human papillomavirus on vaginally inserted sex toys, before and after cleaning, among women who have sex with women and men This finding is relevant for partners who share toys, particularly women who have sex with women, a population sometimes overlooked in STI prevention messaging. The practical advice: use a condom on shared toys, or clean them thoroughly between partners and allow time before reuse.
Trichomoniasis, caused by a protozoan parasite, has also been detected on wet surfaces in laboratory settings, though real-world fomite transmission is considered rare. Scabies mites and pubic lice, both of which are sometimes classified among sexually transmitted infections, spread readily through close physical contact but can also transfer via shared bedding, towels, or clothing. Pubic lice transmission occurs through close body contact, and clinicians are advised to consider the possibility of concomitant STIs when diagnosing an infestation.9Clinical and Experimental Dermatology. A clinical review and history of pubic lice Pubic lice can infest not just pubic hair but also eyelashes, eyebrows, and body hair, which occasionally leads to transmission routes that have nothing to do with sex.
Autoinoculation and Hand-to-Eye Transfer
One often-overlooked pathway is autoinoculation, where a person transfers an infection from one part of their own body to another. Chlamydial conjunctivitis (a chlamydia infection of the eye) in adults is a textbook example. The standard explanation is that infected genital secretions get carried to the eye by the patient’s own hands. A study of 60 confirmed adult chlamydial conjunctivitis cases found that only two of 38 men and none of 22 women had genital symptoms at the time of diagnosis, suggesting that many of these eye infections came from contact with subclinical genital infections that the patients did not even know they had.10PubMed. Genital infection with Chlamydia trachomatis in patients with chlamydial conjunctivitis: unexplained results
Case reports have also documented chlamydial eye infections from direct contact with a partner’s genital fluids, including scenarios where no genital infection was present in the patient themselves, only in their partner. The transfer could happen through hands or more directly.11International Journal of STD & AIDS. Can chlamydial conjunctivitis result from direct ejaculation into the eye? Herpes can similarly spread from a genital sore to the fingers (herpetic whitlow) and from there to the eyes or mouth. Hand hygiene after genital contact is a simple and often neglected step that reduces this type of self-infection.
Why Saliva Doesn’t Spread HIV
One of the most enduring myths is that HIV can spread through kissing, sharing cups, or through saliva in general. The evidence strongly says otherwise, and the reason is more interesting than people assume. Research found that saliva rapidly destroys over 90 percent of blood mononuclear leukocytes (the white blood cells HIV lives in) on contact, reducing HIV multiplication by 10,000-fold or more. The main mechanism behind this is the hypotonicity of saliva, meaning that saliva’s low salt concentration causes infected cells to swell and burst.12PubMed. Why is HIV rarely transmitted by oral secretions? Saliva can disrupt orally shed, infected leukocytes
Beyond that physical destruction, the oral environment has additional defensive layers: low viral concentrations in saliva to begin with, very few of the receptor-bearing target cells that HIV needs to establish infection, plus anti-HIV antibodies and other naturally occurring antiviral factors in saliva that work together to block transmission.13PubMed. The role of the oral environment in HIV-1 transmission This is why oral sex carries a much lower HIV risk than vaginal or anal sex, and casual contact through saliva is essentially a non-risk. None of this means oral sex is zero-risk for other STIs, since herpes, gonorrhea, syphilis, and HPV can all transmit orally. But for HIV specifically, the mouth is a remarkably hostile environment for the virus.
Biological Factors That Change Your Risk
Two people can engage in the same sexual act with the same infected partner and face very different odds of acquiring an infection. Several biological factors shift the probability up or down.
Having an existing STI is one of the most significant risk amplifiers. HSV-2 infection increases the risk of acquiring HIV, and when someone is coinfected with both, it raises the transmission risk for both viruses.14PubMed Central. The population impact of herpes simplex virus type 2 (HSV-2) vaccination on the incidence of HSV-2, HIV and genital ulcer disease in South Africa: a mathematical modelling study The herpes sores create breaks in the mucosal barrier, and the inflammation they produce floods the area with exactly the immune cells that HIV targets. This creates a vicious cycle in populations where both infections are common.
Bacterial vaginosis, a common imbalance in vaginal bacteria that is not itself an STI, has a similar effect. Research found that the prevalence of STIs was significantly higher in women with BV than in those without, and co-infections involving multiple STI-causing organisms were dramatically more frequent in the BV group.15PubMed Central. Relationship between Bacterial Vaginosis and Sexually Transmitted Infections: Coincidence, Consequence or Co-Transmission? BV disrupts the normally acidic vaginal environment and depletes protective Lactobacillus bacteria, leaving the tissue more permeable to incoming pathogens. Whether BV directly causes increased susceptibility, or whether the same behaviors that expose someone to STIs also promote BV, remains an active question, but the association is strong enough to be clinically relevant.
Certain hormonal contraceptives may also alter susceptibility. Research has shown that some injectable progestins increase the permeability of mucosal epithelial tissue by acting on the proteins that hold epithelial cells together, enhancing the ability of infectious agents to penetrate genital tissue.16PubMed Central. Hormonal contraceptive use and the risk of sexually transmitted infections: a systematic review and meta-analysis This does not mean hormonal contraception should be avoided, as the benefits of preventing unintended pregnancy remain substantial, but it underscores that contraceptive counseling and STI prevention are separate conversations that ideally happen together.
Male circumcision appears on the protective side of the ledger. Removal of the foreskin disrupts several of the mechanisms by which HIV gains entry during vaginal intercourse, including reducing the surface area of vulnerable mucosal tissue and decreasing the number of target cells exposed during sex.17The Open AIDS Journal. Male Circumcision and HIV Transmission; What Do We Know? The protective effect has been demonstrated most clearly for heterosexual men acquiring HIV from female partners; the evidence is less robust for other populations and other infections.
How Condoms and Treatment Interact with Transmission Risk
Condoms remain the single most effective barrier against most sexually transmitted infections during intercourse. For HIV specifically, the data is striking: the estimated per-act transmission risk drops by over 99 percent when condoms are combined with antiretroviral treatment of the HIV-positive partner.1PubMed Central. Estimating per-act HIV transmission risk: a systematic review This is the basis for the “undetectable equals untransmittable” (U=U) messaging in HIV prevention: when viral load is suppressed to undetectable levels through treatment, sexual transmission effectively does not occur, and adding a condom pushes residual risk to essentially zero.
Condoms are less effective against infections that spread via skin-to-skin contact in areas the condom doesn’t cover. HPV and herpes are the main examples: both can transmit from skin on the pubic area, inner thighs, or base of the penis that sits outside the condom. Condoms reduce transmission of these infections, but they do not eliminate it the way they can for pathogens that travel strictly in fluids. This is one reason HPV vaccination has become such a cornerstone of prevention strategy for that particular virus.
Pre-exposure prophylaxis (PrEP) for HIV adds another layer. Daily or on-demand PrEP with tenofovir-based medications has been shown to reduce HIV acquisition risk by over 90 percent in clinical trials among adherent users. Newer injectable PrEP formulations given every two months have shown even higher efficacy in some populations. However, PrEP protects only against HIV. People on PrEP who stop using condoms sometimes see increases in other STIs like gonorrhea and syphilis, a trade-off that public health programs try to manage through regular screening bundled with PrEP prescriptions.
Infections That Blur the Line Between STI and Non-STI
The classification of an infection as “sexually transmitted” can be misleading because it implies sex is the only way to catch it. In practice, many infections exist on a spectrum. Hepatitis B is sexually transmitted, but it also spreads through blood and from mother to child, which is why universal infant vaccination programs target it. Hepatitis A outbreaks have occurred through both contaminated food and sexual contact, particularly oral-anal contact. Molluscum contagiosum is a skin virus that can spread through sex or through entirely non-sexual skin contact, shared towels, or wrestling mats.
Pubic lice sit in this gray zone as well. Although usually acquired through sexual contact, they can transfer through shared bedding or clothing, and when they colonize eyelashes in children, sexual abuse is one possible explanation but not the only one.9Clinical and Experimental Dermatology. A clinical review and history of pubic lice The important point for clinicians and patients alike is that an STI diagnosis does not automatically mean sexual contact was the route of transmission in every case, and assumptions about how someone “must have” gotten an infection can be wrong and harmful.
Zika virus, though primarily mosquito-borne, can also transmit sexually and was documented doing so during the 2015–2016 outbreaks. Ebola virus has been found in semen months after clinical recovery. These examples sit at the outer edge of what most people think of as STIs, but they illustrate the broader principle: any pathogen that can survive in genital secretions or blood has at least a theoretical sexual transmission route. The label “sexually transmitted infection” describes a primary route, not an exclusive one, and appreciating that flexibility helps people make better decisions about their own risk.