Sexually transmitted infections spread through three main routes: direct skin-to-skin contact, exchange of bodily fluids during sex, and blood-to-blood contact. More than 1 million curable STIs are acquired every day worldwide among people aged 15 to 49, and many of those infections pass between people who have no visible symptoms at all.
Sexual Contact Is the Primary Route
Most STIs spread during vaginal, anal, or oral sex. But the specific mechanism differs depending on the infection. Some require the exchange of fluids like semen, vaginal secretions, or blood. Others only need direct contact between skin or mucous membranes, even without any fluid exchange.
This distinction matters because it affects how much protection barriers like condoms can offer. Condoms are highly effective against fluid-borne infections, but they cover only the skin they physically touch. An infection that lives on surrounding genital skin can still pass between partners during protected sex.
Skin-to-Skin Infections
Herpes (HSV) and HPV are the two most common STIs that spread primarily through skin-to-skin contact. The virus lives in skin cells and sheds from the surface, so any direct contact with an infected area can transmit it, whether or not a sore, wart, or any other visible sign is present. Syphilis also spreads this way during its early stages, when an open sore called a chancre touches another person’s skin or mucous membranes.
What makes these infections particularly easy to pass along is that transmission doesn’t require penetration. Genital rubbing, hand-to-genital contact, and oral sex can all create enough skin contact. HPV is so efficiently transmitted that most sexually active people will encounter it at some point in their lives.
Fluid-Based Infections
HIV, chlamydia, gonorrhea, and trichomoniasis spread mainly through bodily fluids: semen, pre-seminal fluid, vaginal secretions, rectal fluids, and (for HIV) blood. Semen is the major vector for male-to-female HIV transmission, and it also carries bacteria responsible for gonorrhea and chlamydia. These pathogens need to reach mucous membranes or enter through small breaks in tissue to establish an infection.
Anal sex carries a higher transmission risk for fluid-borne STIs than vaginal sex because the lining of the rectum is thinner and more prone to tiny tears, giving pathogens easier access to the bloodstream. Vaginal sex is the next highest risk. Oral sex carries a lower risk for HIV specifically, though it can still transmit gonorrhea, syphilis, herpes, and chlamydia. Gonorrhea of the throat, for example, is relatively common among people who perform oral sex on a partner with a genital infection.
Transmission Without Symptoms
One of the biggest reasons STIs spread so effectively is that many infected people never know they have one. Chlamydia and gonorrhea frequently cause no symptoms at all, especially in women. Herpes can shed from the skin on days when no sore is visible. Research from Fred Hutch Cancer Center found that people with herpes who had never noticed symptoms still shed the virus on about 10 percent of days tested. Among those who did have recognized symptoms, shedding occurred on about 20 percent of days. Over a two-month study period, 68 percent of people with no history of herpes symptoms still had detectable virus on at least one day.
The practical takeaway: most herpes transmissions come from people who don’t know they carry the virus or who aren’t experiencing an active outbreak. The same principle applies broadly. Chlamydia, gonorrhea, HPV, and even HIV can all be passed during periods when the infected person feels completely fine.
Non-Sexual Routes
Several STIs can also spread outside of sexual activity, primarily through blood contact. Sharing needles or syringes for injecting drugs is a well-established route for HIV, hepatitis B, and hepatitis C. The risk extends to any equipment involved in drug use, including cookers, filters, pipes, and straws used for snorting.
Tattoos, body piercings, and acupuncture done with non-sterile equipment can transmit hepatitis B and C. Sharing personal items like razors, toothbrushes, or nail clippers poses a low but real risk for hepatitis B, which is hardy enough to survive on surfaces for several days. HIV, by contrast, is fragile outside the body. It loses 90 to 99 percent of its ability to infect within several hours on a dry surface, and the virus cannot survive in water. You cannot get HIV from a toilet seat, a swimming pool, or sharing dishes. However, HIV can survive inside a used needle for over a month under the right conditions, which is why needle sharing remains dangerous.
From Parent to Child
A pregnant person with an STI can pass certain infections to their baby during pregnancy, labor, or breastfeeding. Syphilis can cross the placenta and infect a fetus months before birth, potentially causing serious complications including stillbirth. HIV can transmit during delivery or through breast milk. Chlamydia and gonorrhea can pass to a baby during vaginal birth, sometimes causing eye infections or pneumonia.
Routine prenatal screening catches most of these infections early. For HIV, treatment during pregnancy can reduce the viral load low enough that the risk of passing it to the baby drops dramatically. When the virus is not well controlled near the time of delivery, a scheduled cesarean birth may be recommended to minimize the baby’s exposure.
Why Type of Sex Matters
Not all sexual activities carry the same risk. Penetrative anal sex is the highest-risk activity for most fluid-borne STIs, followed by vaginal sex. Oral sex is lower risk for HIV but still a meaningful route for gonorrhea, syphilis, and herpes. Non-penetrative activities like mutual masturbation or genital rubbing carry lower risk overall, though skin-to-skin infections like herpes and HPV can still spread this way.
Comparing exact risk percentages across different activities is difficult because most people don’t limit themselves to one type of sex, making it hard for researchers to isolate the contribution of each. What is clear is that receptive partners (the person being penetrated) generally face higher risk than insertive partners, because mucous membranes in the vagina and rectum are more vulnerable to infection than the skin of the penis.
How to Reduce Risk
Condoms and dental dams reduce transmission of fluid-borne STIs significantly when used consistently and correctly. They are less protective against infections that spread through skin-to-skin contact in areas the barrier doesn’t cover, but they still reduce risk.
Regular testing is one of the most effective tools available, precisely because so many infections cause no symptoms. Getting tested between partners, or at regular intervals if you have multiple partners, catches infections early and prevents unknowing transmission. Vaccines exist for HPV and hepatitis B, both of which are most effective when given before exposure. For people at higher risk of HIV, pre-exposure prophylaxis (PrEP) reduces the chance of infection by more than 99 percent when taken as prescribed.
Reducing the number of sexual partners lowers your statistical exposure, but it doesn’t eliminate risk. A single encounter with an infected partner is enough for transmission. The combination of barrier methods, testing, vaccination, and open communication with partners provides the strongest overall protection.