STIs spread primarily through vaginal, anal, or oral sex, but some can also pass through skin-to-skin contact, shared needles, or from parent to child during pregnancy and birth. The specific route depends on the type of infection, and understanding each one helps you know where your actual risks are.
Vaginal and Anal Sex
Unprotected vaginal or anal sex carries the highest risk for most STIs. These infections travel through semen, vaginal fluids, and blood, meaning any exchange of these fluids between partners creates an opportunity for transmission. Anal sex tends to carry even higher risk than vaginal sex because the lining of the rectum is thinner and more prone to small tears, giving bacteria and viruses a direct route into the bloodstream.
Condoms significantly reduce the risk for infections that spread through fluids, including HIV, gonorrhea, chlamydia, and trichomoniasis. For infections that spread through skin contact in areas a condom doesn’t cover, such as herpes, syphilis, and HPV, protection is real but limited. If the infected skin or sore falls outside the area the condom covers, transmission can still happen.
Oral Sex
Oral sex is lower risk than vaginal or anal sex, but it’s far from risk-free. Chlamydia, gonorrhea, syphilis, herpes, HPV, and HIV can all spread through oral contact with a partner’s genitals. Throat infections from chlamydia or gonorrhea often cause no symptoms, which means a person can unknowingly pass these infections to future partners through oral sex.
Dental dams and condoms reduce the risk during oral sex by preventing direct contact between the mouth and genital or anal tissue. Without a barrier, any cut, sore, or inflamed area in the mouth can serve as an entry point.
Skin-to-Skin Contact
Some STIs don’t need fluid exchange at all. Herpes (HSV) and HPV spread through direct skin-to-skin contact, which is why condoms offer only partial protection against them. If an infected area of skin touches a partner’s skin or mucous membrane, the virus can transfer. This can happen during genital contact, oral sex, or even close skin contact in the genital region that doesn’t involve penetration.
What makes these infections particularly easy to catch is that they can spread even when no visible sore or wart is present. Research from the University of Washington tracked people with genital HSV-1 and found they were shedding virus on about 12% of days in the first few months after infection, dropping to 7% by 11 months. HSV-2 sheds even more frequently: roughly 34% of days in the first year and 17% of days at the 10-year mark. During all of this shedding, most participants had no symptoms at all. A person can genuinely not know they’re contagious.
Shared Needles and Blood
HIV, hepatitis B, and hepatitis C spread efficiently through blood. Sharing needles for drug injection, tattoos, or piercings with contaminated equipment puts you at risk. Even a small amount of infected blood left on a needle is enough. Transmission through blood transfusions is now extremely rare in countries that screen donated blood, but it remains a concern in settings without routine screening.
From Parent to Child
Several STIs can pass from a pregnant person to their baby. This can happen three ways: through the bloodstream across the placenta during pregnancy, through exposure in the birth canal during delivery, or through breastfeeding after birth. HIV, syphilis, herpes, hepatitis B, chlamydia, and gonorrhea can all transmit this way. Routine prenatal screening catches most of these early enough to treat or manage them before delivery, which dramatically reduces the baby’s risk.
Why Many People Don’t Know They Have One
A major reason STIs spread so effectively is that many cause no obvious symptoms, especially early on. Chlamydia and gonorrhea frequently produce no symptoms in women. Herpes can shed virus without causing sores. HPV often clears on its own without a person ever knowing they had it, but can also persist and cause problems years later. Syphilis can produce a painless sore that heals on its own, giving the false impression the infection resolved.
This is why screening matters even when you feel fine. Current guidelines recommend chlamydia and gonorrhea screening annually for sexually active women under 25, and for anyone at increased risk regardless of age. HIV screening is recommended at least once for all adults between 13 and 64. Men who have sex with men are advised to screen for chlamydia, gonorrhea, syphilis, and HIV at least annually, and every three to six months if at higher risk.
Testing Windows After Exposure
If you think you’ve been exposed, getting tested too early can produce a false negative. Each infection has a window period where it won’t show up reliably on tests.
- Chlamydia and gonorrhea: Most tests are accurate within one to two weeks after exposure.
- Syphilis: A blood test catches most infections at one month, and nearly all by three months.
- HIV: A blood test using the newer antigen/antibody method catches most cases at two weeks and nearly all by six weeks. Oral swab tests take longer, with most detected at one month and nearly all by three months.
- Hepatitis B: Blood tests are typically accurate at three to six weeks.
- Hepatitis C: Most infections show up at two months, but it can take up to six months to catch nearly all cases.
If your first test comes back negative but the exposure was recent, a follow-up test after the full window period gives you a more reliable answer.
What Condoms Protect Against, and What They Don’t
Latex and polyurethane condoms are highly effective against HIV and very good at preventing gonorrhea, chlamydia, and trichomoniasis, all of which spread through genital fluids. For infections that spread through skin contact, including herpes, HPV, and syphilis, condoms reduce the risk but can’t eliminate it. The infected skin may simply be in an area the condom doesn’t cover.
HPV vaccines offer strong protection against the strains most likely to cause genital warts and cervical cancer. For people already exposed to herpes or HIV, daily antiviral medications lower the amount of virus in the body and significantly reduce the chance of passing it to a partner. Pre-exposure prophylaxis (PrEP) is available for people at higher risk of HIV and is highly effective when taken as prescribed.