STDs (also called STIs) spread through several distinct routes, not just one. The main pathways are contact with infected bodily fluids, direct skin-to-skin contact, blood exposure, and transmission from mother to baby. The specific route depends on the infection. Some require fluid exchange, others can spread from touching infected skin with no visible symptoms present.
Bodily Fluid Transmission
Many of the most common STIs travel through semen, vaginal secretions, pre-ejaculate, or rectal fluids. HIV, chlamydia, gonorrhea, and hepatitis B all fall into this category. During vaginal, anal, or oral sex, these fluids come into contact with mucous membranes, the thin, absorbent tissue lining the genitals, rectum, and mouth. Pathogens in the fluid can cross through these membranes and establish an infection.
The risk isn’t equal across all types of sex. CDC data on HIV illustrates this clearly: receptive anal sex carries a per-act risk of roughly 1 in 72, while receptive vaginal sex is about 1 in 1,250. Insertive vaginal sex drops further to around 1 in 2,500. These numbers reflect a single encounter without a condom, and the differences come down to biology. The rectal lining is thinner and more prone to small tears, giving the virus easier access to the bloodstream.
Skin-to-Skin Contact
Not all STIs need fluid exchange to spread. Herpes (HSV), HPV, and syphilis can all transmit through direct skin-to-skin contact with an infected area. This is why condoms, while highly effective against fluid-borne infections, offer less protection against these three. A condom only covers part of the genital region, and if the infected skin or a sore sits outside that area, transmission can still occur.
HPV is especially efficient at skin-to-skin spread. It’s the most common sexually transmitted infection in the world, in large part because it passes so easily through contact with infected genital skin, even when no warts or other signs are visible.
Oral Sex Is Not Risk-Free
A common misconception is that oral sex carries little or no STI risk. In reality, chlamydia, gonorrhea, syphilis, herpes, HPV, and HIV can all be transmitted through oral sex. Giving oral sex to someone with a genital or anal infection can lead to an infection in your mouth or throat. Receiving oral sex from someone with a throat infection can lead to a genital infection. You can carry an STI in more than one location at once, for example in both the throat and the genitals.
Oral-anal contact (rimming) adds further risks, including hepatitis A, hepatitis B, and intestinal parasites like Giardia. These spread through trace amounts of fecal matter, even when hygiene seems adequate.
Health guidelines now recommend that clinicians screen for gonorrhea and chlamydia at sites beyond the genitals, including the throat and rectum, particularly in people with relevant exposures. Many throat and rectal infections produce no symptoms at all, which means they often go undiagnosed and continue spreading.
Asymptomatic Transmission
One of the most important things to understand about STI transmission is that it frequently happens when the infected person has no symptoms and no idea they’re contagious. This is particularly well documented with herpes. Up to 70% of herpes transmissions occur during asymptomatic shedding, meaning the virus is present on the skin surface with no visible sores or discomfort.
Nearly all people with genital herpes shed the virus asymptomatically at some point. Studies tracking infected individuals found detectable virus on 8% to 27% of days, with higher rates in the first six months after initial infection. In one study of couples where one partner had herpes and the other didn’t, even with counseling to avoid sex during outbreaks, about 10% of uninfected partners contracted the virus over the course of a year. The majority of those transmissions happened when the infected partner had no outbreak at all.
Chlamydia and gonorrhea also spread readily from people with no symptoms. Chlamydia in particular is often called a “silent” infection because the majority of cases, especially in women, produce no noticeable signs.
Blood and Needle Exposure
HIV, hepatitis B, and hepatitis C can spread through blood-to-blood contact. The most common non-sexual route is sharing needles or syringes, where small amounts of infected blood pass directly into another person’s bloodstream. This applies to injection drug use, and also to any equipment that pierces the skin if not properly sterilized, including tattoo or piercing equipment.
Occupational exposure through accidental needlesticks is possible but extremely rare. Only 58 confirmed cases of occupational HIV transmission to healthcare workers have ever been reported in the United States. Blood transfusions in developed countries carry virtually no risk today due to rigorous screening, though they remain a concern in some parts of the world.
Mother-to-Child Transmission
STIs can pass from a pregnant person to their baby through three different pathways: during pregnancy, during delivery, or during breastfeeding.
Syphilis crosses the placenta and can infect a baby in the womb, sometimes causing serious complications or stillbirth. This risk has driven updated screening recommendations: current guidelines call for syphilis testing three times during pregnancy, at the first prenatal visit, during the third trimester, and at delivery. HIV can also cross the placenta and infect the baby during delivery, though antiretroviral treatment during pregnancy dramatically reduces this risk.
Gonorrhea, chlamydia, hepatitis B, and genital herpes can pass to the baby during vaginal delivery as the infant moves through the birth canal. HIV can also transmit through breast milk, and people with HIV are advised not to breastfeed. With herpes or syphilis, breastfeeding is safe as long as the baby and any pumping equipment don’t touch active sores.
Non-Sexual Spread
A small number of STIs can spread outside of sexual contact entirely. Pubic lice (crabs) can survive up to 24 hours off the body and spread through shared clothing, towels, or bed linens. Hepatitis B is hardy enough to survive on surfaces for days and can transmit through shared razors or toothbrushes that carry trace blood.
Most other STIs, however, do not survive well outside the body. You’re not going to pick up chlamydia from a toilet seat or gonorrhea from a swimming pool. The pathogens that cause these infections need the warmth and moisture of mucous membranes or blood to remain viable.
How Condoms and Barriers Reduce Risk
Latex condoms are highly effective at reducing transmission of STIs that travel through bodily fluids, including HIV, chlamydia, and gonorrhea. They work by physically blocking the exchange of semen, vaginal secretions, and rectal fluids between partners.
For infections spread through skin contact, like herpes, syphilis, and HPV, condoms still help but provide a lower degree of protection. The key factor is whether the condom covers the infected area. If a herpes sore or syphilis chancre is located on skin the condom doesn’t reach, transmission can still happen. Dental dams offer similar partial protection during oral sex by creating a barrier between the mouth and genitals or anus.
Consistent use matters more than occasional use. The protective benefit of condoms in studies comes from data on people who use them every time, not just sometimes. Combining barriers with regular STI testing, particularly for infections that commonly show no symptoms, provides the strongest practical protection.