You can get an STD (also called an STI) through vaginal sex, anal sex, oral sex, skin-to-skin genital contact, shared needles, and in some cases, deep kissing. Most people think of intercourse as the main route, but several common infections spread without penetration and even without fluid exchange. Here’s how each type of contact actually works.
Vaginal and Anal Sex
Penetrative sex carries the widest range of risk. Vaginal intercourse can transmit chlamydia, gonorrhea, hepatitis B, herpes, HIV, HPV, syphilis, trichomoniasis, and mycoplasma genitalium. Anal sex transmits all of those plus hepatitis C. The reason anal sex adds extra risk is that the lining of the rectum is thinner and more prone to small tears, giving infections easier access to the bloodstream.
Some of these infections travel in body fluids like semen and vaginal secretions. Others, including herpes, HPV, and syphilis, spread primarily through direct contact with infected skin or sores. That distinction matters because condoms are more effective against fluid-borne infections (HIV, gonorrhea, chlamydia, trichomoniasis) and less effective against skin-contact infections. HPV and herpes can infect areas a condom doesn’t cover, so condoms reduce but don’t eliminate the risk.
Oral Sex
Oral sex is lower risk than penetrative sex, but it’s far from risk-free. Performing oral sex on a penis can transmit chlamydia, gonorrhea, herpes, HPV, and syphilis to your mouth or throat. Performing oral sex on a vulva can transmit gonorrhea, herpes, HPV, and syphilis. Oral-anal contact (rimming) can pass herpes, HPV, and syphilis, along with hepatitis A and B and intestinal infections like giardia and shigella.
Transmission works in both directions. You can pick up an infection in your throat from a partner’s genitals, and you can pass a throat infection to a partner’s genitals. Syphilis and HIV acquired in the throat spread through the entire body the same way a genital infection would. Certain strains of HPV that infect the throat can, over many years, contribute to cancers of the back of the tongue, tonsils, and throat.
Skin-to-Skin Contact Without Penetration
You don’t need penetration or even fluid exchange to catch some STIs. Herpes, HPV, syphilis, and mpox all spread through close skin-to-skin touching during sexual contact. If your genitals touch a partner’s genitals, even briefly and without intercourse, these infections can pass between you.
HPV is the clearest example. It lives in skin cells rather than in blood or semen, so it spreads whenever infected skin presses against a partner’s skin. This is one reason HPV is so common: roughly 80% of sexually active people encounter it at some point. Syphilis transmits through contact with a syphilitic sore (called a chancre), which can appear on the genitals, anus, lips, or mouth. Herpes passes through contact with an active sore or, in some cases, through “viral shedding” from skin that looks completely normal.
Even hand-to-genital contact (fingering or handjobs) can transmit herpes, HPV, syphilis, and mpox, though this is uncommon. Deep kissing with tongue can spread herpes and mpox, and less commonly gonorrhea, HPV, and syphilis.
Blood-to-Blood Contact
HIV, hepatitis B, and hepatitis C are all carried in blood. The most common non-sexual way these spread is sharing needles or other injection drug equipment. When you inject with a needle someone else has used, even a tiny amount of blood left inside can carry enough virus to cause infection.
Other blood exposures carry risk too, though they’re less common. Sharing razors or toothbrushes with someone who has hepatitis B can transmit the virus if their blood is on the item and it contacts a cut or opening in your skin. Accidental needle sticks (a concern mostly for healthcare workers) can transmit HIV and hepatitis B and C. Any activity that involves intentional cutting or blood contact during sex carries risk for all three of these infections.
From Parent to Child
STIs can pass from a pregnant person to their baby in three ways. Syphilis crosses the placenta during pregnancy, meaning the baby can be infected well before delivery. Gonorrhea, chlamydia, hepatitis B, and genital herpes typically pass to the baby during delivery as the baby moves through the birth canal. HIV can cross the placenta during pregnancy and also transmit during delivery.
After birth, HIV can spread through breastfeeding. Syphilis and herpes can spread during breastfeeding only if the baby’s mouth contacts an active sore on the breast. With proper screening and treatment during pregnancy, the risk of passing most of these infections to a baby drops dramatically.
Shared Personal Items
The vast majority of STIs do not survive on surfaces like toilet seats, towels, or doorknobs. Most bacteria and viruses that cause STIs die quickly outside the human body. However, a few exceptions exist at very low probability.
Molluscum contagiosum, a viral skin infection that causes small bumps, can spread through shared towels, clothing, bar soap, and razors, particularly if someone shaves over active bumps. HPV can theoretically transfer if someone with genital warts shaves over them and you use the same razor on your own genitals. Hepatitis B and C can survive in dried blood for hours to days, so sharing razors or toothbrushes with visible blood on them poses a small risk. These scenarios are uncommon enough that casual household contact is not a realistic worry for most STIs.
What Lowers Your Risk
Condoms and dental dams are the most accessible protection. For infections carried in body fluids (HIV, gonorrhea, chlamydia, trichomoniasis), consistent condom use reduces risk substantially. For infections spread through skin contact (herpes, HPV, syphilis), condoms help but can’t cover all potentially infected skin, so they provide partial protection.
Vaccines exist for two major STIs. The HPV vaccine prevents the strains responsible for most genital warts and HPV-related cancers. The hepatitis B vaccine is highly effective and part of routine childhood immunization in most countries. For HIV, a daily preventive medication called PrEP reduces the risk of infection by over 99% when taken consistently. Regular testing matters too, because many STIs cause no symptoms at all. Chlamydia, gonorrhea, and even HIV can be present for months or years without obvious signs, which means people unknowingly pass them to partners.