Sexually transmitted infections spread when bacteria, viruses, or parasites pass from one person to another, most commonly during vaginal, anal, or oral sex. These pathogens enter the body through tiny, often invisible abrasions in the moist tissue lining the genitals, anus, mouth, or throat. More than 2.2 million cases of chlamydia, gonorrhea, and syphilis were reported in the United States in 2024 alone, and the actual number of infections is far higher since many go undiagnosed.
Understanding the specific routes of transmission helps explain why some infections are easier to catch than others and why prevention methods work better against certain STDs.
Sexual Contact Is the Primary Route
Vaginal and anal sex carry the highest risk for most STDs. During intercourse, pathogens in semen, vaginal fluid, or blood cross into a partner’s body through the delicate mucosal tissue of the penis, vagina, rectum, or cervix. Anal sex tends to carry higher per-act risk than vaginal sex for many infections because rectal tissue is thinner and more prone to small tears.
Oral sex is lower risk overall but far from risk-free. Chlamydia, gonorrhea, syphilis, herpes, HPV, and HIV can all be transmitted through oral contact with the genitals or anus. Gonorrhea infections in the throat deserve special attention: they’re often harder to treat and may spread more easily to partners through oral sex. HPV transmitted to the mouth or throat can, over time, lead to cancers of the throat, base of the tongue, or tonsils. Oral-anal contact (rimming) can also transmit hepatitis A, hepatitis B, and intestinal parasites.
Skin-to-Skin Spread
Not every STD requires an exchange of body fluids. Herpes, HPV, and syphilis spread through direct skin-to-skin contact with an infected area. This is why condoms reduce but don’t eliminate the risk for these infections: the virus or bacteria can live on skin that a condom doesn’t cover, such as the outer vulva, inner thighs, scrotum, or the area around the anus.
HPV is the clearest example. It spreads during vaginal or anal sex but also through close skin-to-skin touching during sexual activity. HPV is extremely common partly because transmission requires nothing more than contact between infected and uninfected skin. In lab studies, HPV has been shown to persist on surfaces for up to seven days at room temperature, though real-world transmission from objects is considered rare compared to direct contact.
Most Infections Show No Symptoms
One of the biggest reasons STDs spread so widely is that the majority of infected people have no idea they’re carrying anything. Research tracking STD diagnoses found that roughly 66% of detected infections were completely asymptomatic. The numbers are striking when broken down by infection:
- Chlamydia: about 72% of cases had no symptoms
- Gonorrhea: about 70% of cases had no symptoms
- Syphilis: about 40% of cases had no symptoms
- Hepatitis C: 100% of the cases detected were asymptomatic
This means waiting for symptoms before getting tested is an unreliable strategy. People transmit infections to partners for weeks, months, or even years without knowing they’re infected. Regular screening is the only way to catch these silent infections.
Blood-to-Blood Transmission
HIV and hepatitis B and C spread through blood contact in addition to sexual routes. The most common non-sexual pathway is sharing needles or syringes for drug injection. Surveys of people who inject drugs have found that over 50% report sharing needles, and difficulty obtaining clean syringes is a major contributing factor.
These viruses can also enter the bloodstream through other equipment used to inject or prepare drugs, as well as through unsterilized tattoo or piercing tools. Blood transfusion is now an extremely rare route in countries with robust screening programs, but it remains a concern in some healthcare settings globally.
From Mother to Child
Several STDs can pass from a pregnant person to their baby during pregnancy, delivery, or breastfeeding. Without treatment, 15 to 30% of babies born to mothers with HIV become infected during pregnancy and delivery, and an additional 5 to 20% become infected through breastfeeding. With modern treatment, this rate drops dramatically, often below 1%.
Syphilis transmission during pregnancy is particularly dangerous. The U.S. reported 3,941 cases of congenital syphilis in 2024. Syphilis passed to a fetus can cause stillbirth, severe birth defects, and organ damage. Routine prenatal screening catches most cases early enough for treatment to protect the baby.
How Well Condoms Work
Male condoms are highly effective against fluid-borne infections but less protective against skin-to-skin infections. The protection varies considerably depending on the STD:
- HIV: condoms reduce transmission by about 85% with consistent use, and offer over 90% protection overall
- Chlamydia: protection ranges from 33 to 90% depending on how consistently they’re used
- Herpes (HSV-2): condoms reduce transmission by roughly 40%, since the virus often lives on skin outside the area a condom covers
These numbers reflect real-world use, where people sometimes use condoms incorrectly or inconsistently. The gap between HIV protection and herpes protection illustrates a core principle: condoms work best against pathogens carried in fluids and are less effective against those that spread through skin contact in surrounding areas.
Less Common Transmission Routes
Some STDs you may not have heard of follow the same patterns. Mycoplasma genitalium, a bacterial infection that’s increasingly recognized as a common STD, spreads through vaginal and anal sex. Like chlamydia and gonorrhea, it can be passed along even when the infected person has no symptoms at all. Researchers are still investigating whether it spreads through oral sex. Having been treated for it once doesn’t make you immune; reinfection happens with new exposures.
Transmission from surfaces, toilet seats, or casual contact like hugging or sharing utensils is not a realistic concern for the vast majority of STDs. These pathogens evolved to survive in the warm, moist environment of human tissue, and most die quickly once exposed to open air or dry surfaces. While HPV can technically survive on surfaces for days in laboratory conditions, documented cases of transmission through objects are exceedingly rare. You don’t catch STDs from doorknobs, swimming pools, or sitting next to someone.