Sexually transmitted infections spread through a few core routes: sexual contact (vaginal, anal, or oral), skin-to-skin touching, blood exposure, and from parent to child during pregnancy or birth. The specific route depends on the pathogen involved. Some infections require direct exchange of bodily fluids, while others can pass through simple skin contact in the genital area.
How Infections Enter the Body
Most STIs exploit the thin, moist tissue lining the genitals, rectum, and mouth. This tissue, called mucous membrane, is far more vulnerable than regular skin. It lacks the tough outer barrier that protects the rest of your body, and in some areas the cells aren’t tightly sealed together. Pathogens can slip between these cells to reach the immune cells and blood vessels underneath.
Inflammation makes this worse. When an existing infection or irritation is present, the body releases signals that loosen the junctions between cells even further, essentially opening the door wider for new pathogens. This is one reason having one STI increases your risk of acquiring another. Tiny tears or abrasions during sex, which are common and often unnoticed, create additional entry points.
Sexual Contact: Vaginal, Anal, and Oral
Vaginal and anal sex are the highest-risk activities for most STIs because they involve sustained contact with mucous membranes and the exchange of genital fluids. Anal sex carries a particularly high transmission risk for HIV. A large systematic review estimated the per-act risk of acquiring HIV through receptive anal sex at roughly 138 per 10,000 exposures to an infected partner, compared to 8 per 10,000 for receptive vaginal sex. The insertive partner faces lower but real risk in both cases: about 11 per 10,000 for anal sex and 4 per 10,000 for vaginal sex.
Oral sex is lower risk for HIV, but it is not low risk across the board. Gonorrhea, syphilis, herpes, and chlamydia all transmit through oral sex. In a study of gay men diagnosed with syphilis, 1 in 5 reported oral sex as their only sexual activity. Gonorrhea infections in the throat are especially worth noting because they can be harder to treat and can spread to partners through oral contact. Chlamydia can also establish throat infections that serve as a source of further transmission.
One complication in understanding oral sex risk is that most people who have oral sex also have vaginal or anal sex, making it difficult for researchers to isolate the risk from one activity alone.
Skin-to-Skin Contact
Not all STIs require fluid exchange. HPV and herpes (HSV) are the two major infections that spread through direct skin-to-skin contact during sexual activity. HPV spreads most commonly during vaginal or anal sex but also transmits through close skin touching in the genital area, even without penetration. This means condoms reduce but don’t eliminate the risk, since they don’t cover all potentially infected skin.
Herpes follows a similar pattern. The virus lives in nerve cells and periodically reactivates at the skin surface, where it can be passed to a partner through direct contact. Critically, most herpes transmission happens when no visible sore is present. Research published in the Journal of the Royal Society Interface found that roughly 80% of viral shedding from genital herpes is asymptomatic. Many transmission events occur early in a shedding episode, before any lesion is detectable, or when ulcers are so small (under 2 millimeters) that the infected person wouldn’t notice them. About 24% of transmission episodes in modeling studies occurred with high viral loads despite no visible ulcer wider than 2 millimeters.
Why Asymptomatic Infections Matter
The herpes data highlights a broader reality: many STIs spread from people who have no idea they’re infected. Chlamydia is asymptomatic in the majority of cases. Gonorrhea often produces no symptoms in women and can silently infect the throat or rectum in anyone. HPV infections are almost always invisible. Syphilis can enter a latent phase where symptoms disappear entirely while the infection remains transmissible.
This is why screening matters more than symptom-watching. Updated clinical guidance now recommends syphilis screening three times during pregnancy regardless of risk factors, and supports screening women for gonorrhea and chlamydia at sites beyond the genitals, including the throat and rectum.
Blood-to-Blood Transmission
HIV, hepatitis B, and hepatitis C are the primary STIs that also spread through blood contact. The most common non-sexual route is sharing needles or syringes, which allows infected blood to pass directly into another person’s bloodstream. All three viruses can be acquired through a needlestick injury or through blood splashing onto mucous membranes, which is why healthcare workers follow strict precautions.
For the general public, the practical concern is shared injection equipment. Even small amounts of blood left in a syringe or on drug preparation surfaces can carry enough virus to cause infection, particularly for hepatitis B, which survives outside the body longer than HIV does.
Transmission From Parent to Child
Several STIs can pass from a pregnant person to their baby during pregnancy, labor, or delivery. HIV can cross the placenta or transmit during birth through exposure to blood and vaginal fluids. Without treatment, transmission rates are significant, but antiretroviral therapy during pregnancy reduces the risk dramatically. Syphilis crosses the placenta and can cause severe complications or stillbirth if untreated, which is why prenatal syphilis screening has been expanded. Hepatitis B commonly transmits during delivery, and newborns are routinely vaccinated within hours of birth for this reason. Herpes poses the greatest risk to newborns during vaginal delivery when the mother has an active outbreak, particularly a first-time infection near delivery.
Surfaces and Objects
STIs are poorly suited to surviving outside the body. Most pathogens present in saliva are inactivated once exposed to air. The bacteria and viruses that cause gonorrhea, chlamydia, and HIV are fragile in open environments and die quickly on surfaces like toilet seats, doorknobs, or shared towels. The practical exception is sex toys: genital fluids and blood on these objects can harbor bacteria and viruses for longer periods, so washing and disinfecting them between uses or between partners is a reasonable precaution.
How Much Condoms Reduce Risk
Condoms are highly effective against infections transmitted through fluids but less effective against those spread by skin contact. When used correctly and consistently, male condoms reduce HIV transmission by approximately 85%, and hepatitis B transmission by about 90%. For gonorrhea, protection exceeds 90% in some studies, though real-world effectiveness ranges from 49% to 75% depending on how consistently they’re used.
For chlamydia, consistent use reduces risk by roughly 50% to 90%, with one clinic-based study finding a 33% reduction in practice. Syphilis risk drops by 50% to 71% with perfect use, but only about 29% with typical use. The gap between perfect and typical use matters enormously: condoms that aren’t used for every act of sex, or that are put on partway through, offer far less protection.
For herpes, condoms reduce transmission by about 40%. For HPV, the reduction is partial because both viruses infect skin that condoms don’t cover. This doesn’t mean condoms aren’t worth using for these infections. A 40% reduction in herpes risk is meaningful over time, and HPV vaccination covers the strains responsible for most cancers and genital warts.
What Makes Transmission More Likely
Several factors increase the odds of any given sexual exposure resulting in infection. Having an existing STI, even an asymptomatic one, creates inflammation that loosens the cellular barriers in genital tissue. A higher viral or bacterial load in the infected partner means more pathogen is present during contact. Receptive anal sex carries higher risk than vaginal sex because the rectal lining is a single cell layer thick and tears more easily. The presence of cuts, sores, or ulcers on either partner provides direct access to the bloodstream.
Newer infections tend to be more transmissible. Someone recently infected with HIV, for instance, has a very high viral load during the first weeks before their immune system mounts a response. Similarly, a first episode of genital herpes involves more prolonged and intense viral shedding than later recurrences.