Sexually transmitted infections spread through two main routes: exchange of bodily fluids during sex and direct skin-to-skin contact with an infected area. Most infections pass between people during vaginal, anal, or oral sex, but the specific mechanism varies by pathogen. Understanding which route applies to which infection helps explain why some STIs are easier to prevent than others.
Fluid-Based Versus Skin-to-Skin Spread
STIs fall into two broad categories based on how they move between people. The first group travels through bodily fluids: semen, vaginal secretions, blood, and sometimes saliva. HIV, chlamydia, gonorrhea, and hepatitis B all require fluid exchange to spread. During vaginal, anal, or oral sex, infected fluid comes into contact with mucous membranes (the thin, moist tissue lining the genitals, rectum, and mouth), and the pathogen enters the body through those surfaces. Anal sex carries a particularly high transmission risk because the rectal lining is thinner and more prone to small tears.
The second group spreads through skin-to-skin contact. HPV, herpes, and syphilis can all pass between people when skin touches an infected area, even without any fluid exchange. HPV is the clearest example. It spreads during close skin-to-skin touching during sex and can infect areas a condom doesn’t cover, like surrounding genital skin. Syphilis transmits through direct contact with a syphilis sore, which can appear on the genitals, anus, rectum, lips, or mouth. Herpes spreads the same way, through contact with an active sore or with skin that is “shedding” the virus between outbreaks.
Some infections use both routes. Herpes, for instance, can spread through both skin contact and fluid from an open sore. This overlap is part of why STIs are so common globally: over one million curable STIs are acquired every day worldwide among people aged 15 to 49, according to the World Health Organization.
Oral Sex Is Not Risk-Free
A common misconception is that oral sex carries little to no STI risk. In reality, the CDC lists chlamydia, gonorrhea, syphilis, herpes, HPV, and HIV as infections that can all pass through oral-genital contact. Gonorrhea and herpes are among the most frequently transmitted this way. Gonorrhea can infect the throat, often without symptoms, and a person with a throat infection can then pass it to a partner’s genitals during oral sex.
HPV transmitted through oral sex is linked to cancers in the back of the throat, including at the base of the tongue and tonsils. These oropharyngeal cancers have risen sharply in recent decades.
Oral-anal contact (sometimes called rimming) carries its own set of risks, including hepatitis A, hepatitis B, and intestinal infections from bacteria like E. coli and Shigella, as well as parasites like Giardia.
Bloodborne Transmission
HIV, hepatitis B, and hepatitis C are the three most common bloodborne STIs. While all three can spread during sex, they also transmit through any activity that involves contact with infected blood. Sharing needles for drug injection is the most well-known non-sexual route. Contaminated tattoo or piercing equipment, accidental needlestick injuries in healthcare settings, and sharing personal items like razors that may carry trace amounts of blood are less common but documented pathways.
Hepatitis B is especially resilient. The virus can survive outside the body for at least seven days on surfaces, remaining infectious during that time. HIV, by contrast, is fragile and dies quickly once exposed to air.
Spread Without Symptoms
The majority of new STI cases worldwide are transmitted by people who have no symptoms. This is one of the most important facts about STI transmission and the one most often overlooked. Chlamydia is a textbook example: roughly 70% of women and 50% of men with chlamydia have no noticeable symptoms. Gonorrhea follows a similar pattern, particularly in throat and rectal infections, which are frequently silent.
Herpes spreads between outbreaks through a process called asymptomatic shedding, where the virus periodically becomes active on the skin surface without causing visible sores. A person can transmit herpes to a partner during these periods without either person realizing it. HPV works similarly. Most people with HPV never develop warts or any other visible sign, yet they can still pass the virus through skin contact.
This is why regular screening matters so much. Waiting for symptoms before getting tested means many infections go undetected and continue spreading.
Transmission From Parent to Child
Several STIs can pass from a pregnant person to their baby through three different routes. During pregnancy, infections like syphilis and HIV can cross the placenta and reach the fetus through the bloodstream. During delivery, the baby can pick up infections like herpes, gonorrhea, and chlamydia while passing through the birth canal. After birth, HIV and hepatitis B can transmit through breastfeeding, either through breast milk or blood from cracked nipples.
The consequences can be severe. Syphilis transmitted during pregnancy can cause stillbirth, serious birth defects, or neonatal death. Herpes acquired during delivery can lead to life-threatening infections in newborns. Routine prenatal STI screening exists specifically to catch and treat these infections before they reach the baby.
Why Condoms Help but Don’t Eliminate Risk
Condoms are highly effective at preventing HIV and reducing the risk of fluid-borne infections like chlamydia and gonorrhea. For skin-to-skin infections, however, they provide less protection. The CDC notes that condoms offer limited protection against syphilis and herpes because these infections can transmit through contact with skin that the condom doesn’t cover. HPV falls into the same category: condoms reduce the risk but can’t block it entirely since HPV can infect surrounding genital skin.
This doesn’t mean condoms aren’t worth using for these infections. Consistent condom use does lower transmission rates for herpes and HPV, and it meaningfully reduces the risk of HPV-associated diseases like genital warts and cervical cancer. The protection just isn’t as complete as it is for HIV or gonorrhea.
Dental dams provide a barrier during oral sex, and the HPV vaccine prevents the strains responsible for most HPV-related cancers and genital warts. Using condoms, getting vaccinated, and getting tested regularly together cover significantly more ground than any single strategy alone.
Surfaces, Toilet Seats, and Shared Objects
Most STIs die quickly outside the human body and are not spread through casual contact like sharing a toilet seat, swimming pool, or doorknob. You won’t catch chlamydia, gonorrhea, or HIV from a surface. HPV is one notable exception in lab settings: studies have shown that HPV can persist on surfaces for up to seven days at room temperature. However, even with this durability, documented cases of non-sexual HPV transmission through objects are rare, and the practical risk from surfaces remains extremely low compared to direct contact during sex.
Trichomoniasis, a common parasitic STI, can theoretically survive briefly on moist objects like wet towels, but transmission through shared objects is not considered a meaningful route. The overwhelming majority of trichomoniasis cases result from sexual contact.