You can get a sexually transmitted infection (STI) through vaginal, anal, or oral sex, through skin-to-skin genital contact, and in some cases through shared needles, childbirth, or contaminated personal items. Most people picture penetrative sex as the only route, but several common infections spread without it. More than 2.2 million STIs were reported in the United States in 2024 alone, and the World Health Organization estimates that over 1 million curable STIs are acquired every day worldwide, the majority of them in people who have no symptoms at all.
Vaginal and Anal Sex
Vaginal and anal intercourse are the highest-risk activities for STI transmission. During these acts, infected body fluids like semen, vaginal secretions, and blood come into direct contact with mucous membranes, the thin, moist tissue lining the genitals, rectum, and cervix. These membranes absorb pathogens far more easily than regular skin does. Chlamydia, gonorrhea, syphilis, HIV, hepatitis B, and trichomoniasis all spread this way.
Anal sex carries particular risk because rectal tissue is thinner and more prone to tiny tears, giving infections an easier entry point. The risk of HIV transmission, for example, is higher during anal sex than vaginal sex. Any open sore or cut in the genital or anal area further increases vulnerability to virtually every STI.
Oral Sex
Oral sex is lower risk than vaginal or anal sex for most infections, but it is not risk-free. The CDC lists chlamydia, gonorrhea, syphilis, herpes (both types), HPV, and HIV as infections that can pass through oral-genital contact. Oral contact with the anus can also transmit hepatitis A and B, along with intestinal parasites.
The risk of getting HIV specifically from oral sex is much lower than from anal or vaginal sex. That may not hold true for other infections, though. Gonorrhea of the throat, for instance, is relatively common and often causes no symptoms, meaning it can spread further without anyone realizing it. Sores, cuts, or bleeding gums in the mouth raise the odds of both giving and receiving an infection during oral sex.
Skin-to-Skin Contact
Not every STI requires the exchange of body fluids. HPV and herpes (HSV) both spread through direct skin-to-skin touching during sexual activity, even without penetration. HPV is most commonly spread during vaginal or anal sex but also transmits through close skin-to-skin contact during sex. Herpes spreads from active sores or from skin that is “shedding” the virus without any visible outbreak.
This matters because condoms, while highly effective against fluid-borne infections, offer less protection against these two. They only block contact with the skin they cover. Infected skin on the thighs, groin, or areas around the genitals that a condom doesn’t reach can still transmit HPV or herpes. Syphilis, which causes open sores called chancres, similarly spreads through direct contact with those sores, whether or not they’re in an area a condom covers.
Shared Needles and Blood Exposure
HIV, hepatitis B, and hepatitis C can all spread through blood. Sharing needles or syringes for drug injection is the most common non-sexual route. Even small amounts of blood left in a needle or on injection equipment can carry enough virus to cause infection. Sharing equipment for tattooing or piercing in unregulated settings poses a similar risk.
Blood transfusions were once a significant source of HIV and hepatitis transmission, but rigorous screening of donated blood has made this route extremely rare in countries with modern blood banking systems.
Mother-to-Child Transmission
Several STIs can pass from a pregnant person to their baby during pregnancy, labor, or breastfeeding. HIV can transmit at any of these stages, though treatment during pregnancy dramatically reduces the odds. Syphilis can cross the placenta and cause congenital syphilis, which led to thousands of cases in the U.S. in 2024. Active genital herpes lesions are contagious during childbirth, which is why a cesarean delivery is often recommended when an outbreak is present at the time of labor. Chlamydia and gonorrhea can infect a newborn’s eyes during vaginal delivery.
Shared Personal Items
For most STIs, you cannot get infected from a toilet seat, doorknob, or swimming pool. The organisms that cause chlamydia, gonorrhea, syphilis, and HIV are fragile outside the body and die quickly on surfaces. However, a few exceptions exist. Pubic lice (“crabs”) can occasionally spread through shared clothing, bedding, or towels, though they need a blood meal and die within 24 to 48 hours without one. Trichomoniasis, caused by a parasite, can theoretically survive briefly on damp surfaces like wet towels, though this route is considered uncommon.
The practical takeaway: surface transmission is not something most people need to worry about. The vast majority of STIs require direct human-to-human contact.
Why Asymptomatic Spread Matters
One of the biggest reasons STIs spread so effectively is that most infected people don’t know they’re infected. The majority of STIs are asymptomatic, meaning they produce no noticeable symptoms at all. Chlamydia is a classic example: many people carry it for months without pain, discharge, or any other sign. Gonorrhea of the throat is often silent. Herpes can shed from the skin between outbreaks. HPV rarely causes symptoms until it has already been transmitted to partners or, in some cases, progressed to precancerous changes.
This is why STIs spread even among people who feel perfectly healthy and would never suspect they’re carrying an infection. Regular screening is the only reliable way to catch these silent infections, especially if you have new or multiple partners.
How Condoms and Barriers Reduce Risk
Latex condoms, when used consistently and correctly, are highly effective at preventing infections that travel through body fluids, including HIV, gonorrhea, chlamydia, and trichomoniasis. They provide a physical barrier that keeps semen, vaginal fluid, and blood from contacting a partner’s mucous membranes.
For infections that spread through skin contact, like herpes, HPV, and syphilis, condoms still reduce risk but offer less protection because they don’t cover all potentially infected skin. Dental dams (thin sheets of latex placed over the vulva or anus) serve a similar barrier function during oral sex, though they’re used far less frequently in practice.
HPV vaccines offer strong protection against the strains most likely to cause genital warts and cancer. For HIV, pre-exposure prophylaxis (PrEP) is a daily or on-demand medication that reduces the risk of infection by over 99% when taken as directed. These tools work best in combination: no single method eliminates all risk, but layering condoms, vaccines, medication, and regular testing together gets close.