STDs (also called STIs) spread through a few specific routes: sexual contact with an infected person, direct skin-to-skin contact, shared needles, and from a pregnant person to their baby. Most people think of vaginal sex as the primary risk, but oral sex, anal sex, and even genital touching without penetration can transmit infections depending on the type.
Vaginal, Anal, and Oral Sex
The most common way STIs spread is through sexual contact involving body fluids or mucous membranes. Vaginal and anal sex carry the highest risk because they involve direct contact with genital fluids and thin, sensitive tissue that infections can easily penetrate. Anal sex is particularly high-risk because the lining of the rectum tears more easily than vaginal tissue, creating a direct path for bacteria and viruses to enter the bloodstream.
Oral sex is lower risk than vaginal or anal sex, but it still transmits several infections. Herpes, syphilis, gonorrhea, and HPV can all pass through oral contact with the genitals or anus. Your risk goes up if you have bleeding gums, gum disease, tooth decay, or open sores in your mouth. Exposure to pre-ejaculate or ejaculate also increases the chance of transmission. Oral-anal contact specifically can transmit hepatitis A, hepatitis B, and intestinal infections.
Skin-to-Skin Contact
Not all STIs require fluid exchange. Herpes and HPV spread through direct skin-to-skin contact, meaning you can get them even without penetration and even when a condom is used. Syphilis also spreads this way during its early stages, when open sores (called chancres) are present. All it takes is your skin touching an infected area on someone else’s body. This is why these infections are so common: herpes and HPV can transmit during foreplay, genital rubbing, or any intimate contact where skin meets skin.
Shared Needles and Blood Contact
HIV, hepatitis B, and hepatitis C all spread through blood-to-blood contact. The most common non-sexual route is sharing needles used to inject drugs. Contaminated equipment for tattoos or body piercings also carries risk, particularly in unregulated settings. Hepatitis C is most commonly transmitted this way rather than through sex. If you share any equipment that breaks the skin, even briefly, and that equipment has been used by someone with an active infection, the virus can enter your bloodstream.
From Parent to Baby
Several STIs can pass from a pregnant person to their baby during pregnancy, delivery, or breastfeeding. HIV can transmit at any of these stages. Gonorrhea, chlamydia, syphilis, and herpes can pass to the baby during vaginal delivery. This is why hospitals routinely apply antibiotic ointment to newborns’ eyes shortly after birth to prevent blindness from undetected gonorrhea or chlamydia exposure. When a pregnant person has active herpes lesions or unsuppressed HIV, a cesarean delivery is typically recommended to reduce transmission risk.
What Doesn’t Spread STIs
The bacteria, viruses, and parasites that cause STIs generally cannot survive on hard surfaces like toilet seats, doorknobs, or shared towels. Bacterial STIs die quickly outside the body. Some viral STIs like herpes and hepatitis B can survive outside the body for a very limited time, but the chance of picking up an infection from a surface is essentially zero. You also cannot get an STI from swimming pools, hot tubs, or casual contact like hugging or sharing food.
Why Condoms Help but Aren’t Perfect
Condoms are highly effective against STIs that spread through genital fluids, including HIV, gonorrhea, chlamydia, and trichomoniasis. They provide a physical barrier that blocks the fluids carrying these infections. For skin-to-skin infections like herpes, HPV, and syphilis, condoms offer less protection because the infected skin or sores may be on areas the condom doesn’t cover. They still reduce risk, but they can’t eliminate it for these infections the way they can for fluid-borne ones.
How Soon Infections Show Up
One reason STIs spread so effectively is that many people don’t know they’re infected. Most STIs have an incubation period where you’re potentially contagious but wouldn’t test positive yet, and many never cause obvious symptoms at all. If you’ve had a possible exposure, here’s how long to wait before testing will be reliable:
- Chlamydia and gonorrhea: One week catches most cases. Two weeks catches nearly all.
- Trichomoniasis: One week catches most. A full month to catch 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.
- HPV: Three weeks to a few months for cervical screening. There is no general screening test for genital or anal warts.
Testing too early can give you a false negative, so timing matters. If your first test comes back negative but you had a clear exposure, retesting after the full window period gives you a more definitive answer. Many STIs, particularly chlamydia and gonorrhea, cause no symptoms at all in a large percentage of people, which is why routine screening after new partners or unprotected sex is the most reliable way to catch infections early.