STDs (also called STIs) spread primarily through vaginal, oral, and anal sex, but the specific route depends on the infection. Some require an exchange of body fluids like semen, vaginal fluid, or blood. Others pass through direct skin-to-skin contact, no fluids needed. Understanding which type you’re dealing with changes everything about how transmission works and how effectively you can prevent it.
Fluid-Based Transmission
Infections like chlamydia, gonorrhea, and HIV travel in body fluids: semen, pre-ejaculate, vaginal secretions, rectal fluids, blood, and breast milk. For transmission to happen, infected fluid needs to reach a mucous membrane (the moist lining inside the vagina, rectum, urethra, or mouth) or enter through a break in the skin. This is why condoms are highly effective against these infections. They physically block the fluid exchange.
Not all sexual acts carry the same level of risk. Receptive anal sex without a condom carries the highest per-act risk for HIV at roughly 1.38%, or about 1 in 72 exposures. Receptive vaginal sex is lower at about 0.08% (1 in 1,234). Oral sex carries the lowest risk, with estimates for HIV ranging from effectively 0% to 0.04% per act. These numbers are per single encounter. The risk compounds over repeated exposures.
Several factors push those numbers higher. Having an existing STI, particularly one that causes open sores, more than doubles the chance of picking up HIV during the same encounter. Bleeding gums, cuts in the mouth, or sores on the genitals all create entry points that wouldn’t otherwise exist. This is why the NHS recommends avoiding brushing or flossing right before oral sex, since minor gum bleeding opens a direct path for pathogens.
Skin-to-Skin Transmission
Herpes (HSV) and HPV don’t need fluid exchange at all. They live in skin cells and spread through direct contact with infected skin or mucosal surfaces. For herpes, the greatest risk is during an active outbreak when sores are visible, but the virus also sheds from normal-looking skin. Studies from Johns Hopkins show that people with genital herpes (HSV-2) shed the virus asymptomatically 1% to 3% of the time. Many new herpes infections come from partners who had no idea they were contagious at that moment.
This is also why condoms offer less protection against skin-to-skin STIs. They cover the shaft of the penis but leave surrounding genital and anal skin exposed. Herpes and HPV can transmit through any contact with those uncovered areas. Condoms reduce the risk but don’t eliminate it the way they can for fluid-borne infections like gonorrhea or chlamydia.
Herpes also crosses between body sites. A cold sore (typically HSV-1) on someone’s mouth can cause a genital herpes infection in their partner during oral sex, and vice versa. The virus doesn’t care about the original location. It infects wherever it finds suitable tissue.
Oral Sex Is Not Risk-Free
A common misconception is that oral sex is “safe” sex. It’s lower risk than vaginal or anal sex, but it can still transmit chlamydia, gonorrhea, syphilis, herpes, HPV, and HIV. Throat infections from gonorrhea and chlamydia are more common than most people realize, and they often cause no symptoms, which means they go untreated and continue spreading.
Giving oral sex generally carries more risk than receiving it, because the person performing oral sex has more direct contact with genital fluids and skin. Factors that increase the risk include poor oral health, gum disease, mouth sores, and exposure to ejaculate. That said, researchers note these risk factors haven’t been precisely quantified in clinical studies.
Why Anal Sex Carries Higher Risk
The lining of the rectum is thinner and more fragile than vaginal tissue. It tears more easily during sex, creating direct access points for pathogens to enter the bloodstream. This is the main reason receptive anal sex has the highest per-act transmission rate for HIV, roughly 17 times higher than receptive vaginal sex. The same vulnerability applies to other STIs. Gonorrhea, chlamydia, herpes, syphilis, and HPV all transmit efficiently through anal sex.
Blood-to-Blood Transmission
HIV, hepatitis B, and hepatitis C spread through blood contact. Sharing needles is the most common non-sexual route: the risk of HIV from a shared needle is about 0.63% per use (1 in 158), while hepatitis B is dramatically higher at 6% to 30%, and hepatitis C sits around 1.8%. The risk depends on how much blood is involved, how recently the needle was used, and how much virus the infected person is carrying.
HIV grown in lab conditions loses 90% to 99% of its ability to infect within hours on an exposed surface, and it cannot survive in water. But inside a used needle, where blood is shielded from air and light, one study found HIV can remain viable for over a month under the right conditions. Hepatitis B is even hardier outside the body.
Blood transfusions with contaminated blood carry an overwhelming 92.5% transmission rate for HIV, but modern screening has made this extraordinarily rare in countries with established blood banking systems.
Mother-to-Child Transmission
STIs can pass from a pregnant person to their baby through three routes. The first is across the placenta during pregnancy, where pathogens in the mother’s bloodstream cross into the fetal blood supply. The second is during delivery, when the baby passes through the birth canal and contacts infected fluids or sores. The third is after birth through breastfeeding, either via breast milk or blood from cracked nipples.
Without any preventive treatment, the chance of passing HIV to a baby during pregnancy and childbirth is about 22.6%, or roughly 1 in 4. With effective treatment that suppresses the virus to undetectable levels, that risk drops to 0.14%. Infections passed earlier in pregnancy tend to cause more severe outcomes, including pregnancy loss or lifelong conditions in the baby. Syphilis, herpes, gonorrhea, chlamydia, hepatitis B, and HIV can all be transmitted this way, which is why STI screening is a standard part of prenatal care.
Asymptomatic Transmission
One of the most important things to understand about STI transmission is that most of it happens when the infected person has no symptoms. Many STIs, including chlamydia, gonorrhea, HPV, and herpes, can be present and contagious without causing any noticeable signs. A person with herpes who is shedding the virus can be fully contagious without a single visible sore. Someone with chlamydia may have no discharge, no pain, and no idea they’re infected.
This is exactly why STIs spread so effectively. People who feel fine and look fine assume they’re not contagious. Regular testing is the only reliable way to know your status, since symptoms alone are a poor indicator of whether someone can transmit an infection.
What Doesn’t Transmit STIs
STI pathogens are fragile outside the human body. You cannot catch an STI from a toilet seat, a swimming pool, a hot tub, sharing food, or casual contact like hugging or handshakes. These organisms need the warm, moist environment of mucous membranes or blood to survive. Surface contact in everyday settings simply doesn’t provide the conditions they require to remain infectious and reach a new host.