Sexually transmitted infections spread through two main routes: the exchange of body fluids during sexual contact and direct skin-to-skin contact with an infected area. But sex isn’t the only way. Some infections also pass from parent to child during pregnancy or birth, through shared needles, and in rare cases through contaminated personal items. Here’s how each route works and what actually raises or lowers your risk.
Vaginal and Anal Sex
Penetrative sex is the highest-risk activity for most STIs. Infections like chlamydia, gonorrhea, HIV, and hepatitis B travel through semen, vaginal fluid, and blood. When these fluids contact the mucous membranes lining the vagina, rectum, or urethra, bacteria and viruses can enter the body easily. Anal sex carries a particularly high risk because the rectal lining is thinner and more prone to small tears, giving pathogens a direct path into the bloodstream.
Other infections don’t need fluids at all. Herpes (HSV) and HPV spread through skin-to-skin contact with an infected area, which means condoms reduce but don’t eliminate the risk since they don’t cover all exposed skin. Syphilis works similarly: it spreads through direct contact with a syphilis sore, which can appear on the genitals, anus, or mouth.
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 be transmitted this way. Oral-anal contact can also transmit hepatitis A and B, as well as intestinal parasites.
Your risk goes up if you have bleeding gums, gum disease, tooth decay, or any sores in or around the mouth. These create openings where pathogens can enter. For the same reason, brushing or flossing right before oral sex is a bad idea, since it can cause tiny cuts in the gums. Generally, the person giving oral sex faces a higher risk than the person receiving it, because they have more direct exposure to genital fluids and skin.
If you have a cold sore (oral herpes, HSV-1) and give someone oral sex, you can transmit herpes to their genitals. The reverse is also true: genital herpes can pass to a partner’s mouth during oral sex.
Why You Can’t Always Tell
One of the biggest reasons STIs spread so effectively is that many infections cause no symptoms at all. More than half of gonorrhea, trichomoniasis, and chlamydia infections can be completely asymptomatic. A person can carry and transmit these infections for weeks or months without knowing anything is wrong. Herpes is another common example: many people with HSV never develop visible sores but can still shed the virus and pass it to partners.
This is why relying on visible symptoms to gauge risk is unreliable. Someone who looks and feels perfectly healthy can still transmit an infection.
From Parent to Child
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. About 40% of babies born to women with untreated syphilis are stillborn or die shortly after birth. Hepatitis C passes to roughly 6 out of every 100 babies born to infected mothers, with the risk climbing higher if the mother also has HIV.
Herpes can also be transmitted during vaginal delivery if the mother has active sores, potentially causing life-threatening infection in the newborn. This is why prenatal STI screening is routine: catching and treating infections early in pregnancy dramatically reduces the risk to the baby.
Shared Needles and Contaminated Items
HIV, hepatitis B, and hepatitis C are bloodborne infections that spread efficiently through shared needles and syringes. This applies not only to injection drug use but also to sharing blood glucose monitoring equipment or any device that pierces the skin.
A few other non-sexual routes exist, though they’re less common. Sharing unwashed sex toys can transmit trichomoniasis, gonorrhea, herpes, HPV, and chlamydia, since these organisms can survive briefly on surfaces. The parasite that causes trichomoniasis can also survive on damp fabrics like towels. Pubic lice (“crabs”) spread through infested bedding, clothing, or towels. And while getting an infection from a toilet seat is technically possible, it’s extremely rare.
What Condoms Do and Don’t Prevent
Condoms are highly effective at blocking infections that travel through genital fluids. Consistent condom use significantly reduces the risk of HIV, gonorrhea, and chlamydia. Lab testing shows that latex condoms provide an effective barrier against even the smallest known STI pathogens.
Where condoms fall short is with infections spread by skin-to-skin contact. Because herpes sores, HPV-infected skin, and syphilis sores can appear on areas a condom doesn’t cover (the outer labia, inner thighs, scrotum, or pubic region), condoms reduce but don’t eliminate transmission of these infections. If you’re using condoms, avoid ones with spermicide, which can actually increase the risk of HIV transmission by irritating mucous membranes.
Testing Windows After Exposure
If you think you’ve been exposed to an STI, timing matters for testing. Tests taken too early can miss an infection that hasn’t built up enough in the body to be detected. Here’s how long to wait after a potential exposure:
- Chlamydia and gonorrhea: Detectable within 1 week for most cases, 2 weeks catches nearly all.
- HIV (blood test): 2 weeks catches most infections, 6 weeks catches almost all.
- Syphilis: 1 month catches most, 3 months catches almost all.
- Herpes (blood test): 1 month catches most, 4 months catches almost all.
- Trichomoniasis: 1 week catches most, 1 month catches almost all.
- Hepatitis B: 3 to 6 weeks.
- Hepatitis C: 2 months catches most, 6 months catches almost all.
Some infections have no screening test at all. Genital warts, anal warts, molluscum contagiosum, and pubic lice are diagnosed by visual inspection only, so you’d need to wait until symptoms appear. For HPV in the cervix, a Pap smear can detect changes starting around 3 weeks to a few months after exposure.
If your first test comes back negative but you’re still within the early window, a follow-up test at the longer time point can confirm the result.