Sexually transmitted infections spread primarily through vaginal, anal, and oral sex. But the specific way each infection passes between people varies. Some require an exchange of body fluids like semen or vaginal secretions, others spread through simple skin-to-skin contact, and a few can pass from a pregnant parent to their baby. Understanding the different routes helps you know what actually puts you at risk.
Vaginal, Anal, and Oral Sex
Sexual contact is by far the most common way STIs spread. During vaginal or anal sex, infections travel through body fluids (semen, pre-seminal fluid, vaginal secretions, or blood) or through direct contact between mucous membranes, which are the soft, moist tissues lining the genitals, rectum, and mouth. Anal sex carries a particularly high transmission risk because the lining of the rectum is thin and tears easily, creating entry points for infection.
Oral sex is lower risk than vaginal or anal sex for most infections, but it’s far from risk-free. Chlamydia, gonorrhea, syphilis, herpes, and HPV can all spread through oral contact with a partner’s genitals. HIV transmission from oral sex is extremely low, though not zero. Oral-anal contact can also transmit hepatitis A, hepatitis B, and intestinal parasites.
Skin-to-Skin Contact
Not every STI needs fluid exchange to spread. Herpes and HPV pass through direct skin-to-skin contact, meaning they can transmit even without intercourse. If your skin touches an area where the virus is active, whether that’s a visible sore, a wart, or skin that looks completely normal, the infection can pass to you. This is one reason condoms reduce but don’t eliminate the risk of these infections: they only cover the skin they physically cover, and herpes or HPV can be present on surrounding areas.
Syphilis also spreads through contact with a syphilis sore, which can appear on the genitals, anus, rectum, lips, or mouth. Heavy petting and other intimate physical contact can occasionally transmit these skin-contact infections, though it’s uncommon.
Many Carriers Have No Symptoms
One of the biggest reasons STIs spread so effectively is that most people who carry them don’t know it. The asymptomatic rates are striking: about 70% of women and 50% of men with chlamydia have no symptoms at all. For gonorrhea, at least half of women and up to 40% of men show no signs. Roughly 70% of people with herpes don’t realize they’re infected, and 70 to 90% of people with HPV never develop visible symptoms.
This means you can’t rely on a partner looking or feeling healthy as any indication of their STI status. Someone with no sores, no discharge, and no discomfort can still pass an infection to you. Herpes, for example, can shed virus from the skin even between outbreaks, a process called asymptomatic shedding. Regular testing is the only reliable way to know.
From Parent to Baby
Several STIs can pass from a pregnant person to their baby during pregnancy, labor, or delivery. The consequences can be severe. Without preventive treatment, up to 90% of infants born to mothers with hepatitis B become infected, and 90% of those infants go on to develop chronic infection. About 40% of babies born to women with untreated syphilis are stillborn or die shortly after birth. Surviving babies can face bone damage, blindness, deafness, and intellectual disability if not treated within the first three months of life.
About 6 in 100 infants born to mothers with hepatitis C become infected, with the risk climbing if the mother also has HIV. HIV itself can pass during pregnancy, delivery, or breastfeeding, though modern treatment has made mother-to-child transmission rare when the infection is known and managed. Standard prenatal care includes STI screening for exactly this reason.
Shared Needles and Blood Exposure
HIV, hepatitis B, and hepatitis C all live in blood. Sharing needles for drug injection, tattooing, or piercing creates a direct blood-to-blood pathway for these viruses. This is one of the primary non-sexual transmission routes and a major reason needle exchange programs exist. Any situation where someone else’s blood could enter your body through a cut, open wound, or mucous membrane carries at least a theoretical risk.
Shared Personal Items
The risk of catching an STI from a towel, razor, or other personal item is very low, but not zero in specific situations. If someone with hepatitis B or C has blood on their razor and you use it while you have a cut, there’s a small chance of transmission. HPV can potentially spread if someone shaves over genital warts and you then use that razor on your own genitals. Molluscum contagiosum, a viral skin infection, can spread through towels, clothing, or other objects that touched an active outbreak. In general, though, STIs don’t survive well on surfaces, and casual sharing of household items is not a meaningful risk for most infections.
How Condoms and PrEP Reduce Risk
Condoms are highly effective against STIs that spread through body fluids, including HIV, chlamydia, and gonorrhea. Lab studies confirm that latex condoms block even the smallest known STI pathogens. However, condoms are less effective against infections that spread through skin-to-skin contact, like herpes, HPV, and syphilis, because these infections can be present on skin that the condom doesn’t cover.
For HIV specifically, PrEP (pre-exposure prophylaxis) reduces the risk of infection from sex by about 99% when taken as prescribed. HPV vaccines are available and highly effective at preventing the strains most likely to cause cancer and genital warts. These tools work best in combination: condoms plus vaccination plus regular testing covers far more ground than any single strategy alone.
When to Get Tested After Exposure
If you’ve had a potential exposure, testing too early can give you a false negative. Every infection has a window period between exposure and when it shows up on a test. Here’s roughly how long you need to wait for reliable results:
- Chlamydia and gonorrhea: one week catches most cases, two weeks catches nearly all
- Syphilis: one month catches most, three months catches nearly all
- HIV (blood test): two weeks catches most, six weeks catches nearly all
- Herpes (blood test): one month catches most, four months catches nearly all
- Hepatitis B: three to six weeks
- Hepatitis C: two months catches most, six months catches nearly all
- Trichomonas: one week catches most, one month catches nearly all
HPV has no routine screening test for most people. It’s detected through Pap smears in people with a cervix, typically starting at age 25, or visually if warts appear. Genital and anal warts, molluscum contagiosum, and pubic lice also have no standard screening tests and are diagnosed by visual examination.
If your first test after exposure comes back negative but it was done on the early end of the window, a follow-up test at the later end of that range gives you more certainty.