STIs spread primarily through vaginal, oral, and anal sex. Some also pass through skin-to-skin contact, shared needles, or from a pregnant person to their baby. Globally, over one million new cases of curable STIs occur every day among adults, adding up to roughly 374 million new infections of chlamydia, gonorrhea, syphilis, and trichomoniasis each year.
Sexual Contact Is the Primary Route
The most common way people get an STI is through vaginal, oral, or anal sex with someone who already has one. During sex, infections travel through body fluids like semen, vaginal secretions, and blood, or through direct contact with infected skin or mucous membranes. You don’t need to have penetrative sex to be at risk. Any sexual contact involving the penis, vagina, mouth, or anus can transmit certain infections.
Intimate physical contact like heavy petting can occasionally spread an STI, though this is uncommon. The key factor is whether infected skin, sores, or fluids come into contact with another person’s body, particularly through mucous membranes (the moist linings of the mouth, genitals, and rectum), which absorb pathogens far more easily than regular skin.
Skin-to-Skin STIs Don’t Require Fluids
Not every STI needs an exchange of body fluids. Herpes and HPV (human papillomavirus) spread through direct skin-to-skin contact. This means condoms reduce the risk but don’t eliminate it completely, since these infections can live on areas of skin that a condom doesn’t cover. Someone with herpes can transmit the virus even when they have no visible sores, a process called asymptomatic shedding. HPV works similarly, often causing no symptoms at all while still being contagious.
Syphilis spreads through contact with syphilis sores, which can appear on the genitals, anus, rectum, lips, or mouth. These sores are painless in the early stage, so many people don’t realize they have one or that they’re exposing a partner.
Oral Sex Carries Real Risk
A common misconception is that oral sex is “safe” sex. It’s lower risk than vaginal or anal sex for most infections, but it can still transmit chlamydia, gonorrhea, syphilis, herpes, HPV, and HIV. Gonorrhea of the throat, for example, is frequently picked up through oral sex and often causes no symptoms, meaning people unknowingly carry and spread it.
Oral contact with the anus adds another layer of risk. This route can transmit hepatitis A and B, as well as intestinal parasites and bacteria like E. coli and Shigella that cause serious gastrointestinal illness.
Transmission During Pregnancy and Birth
Some STIs pass from a pregnant person to their baby before or during delivery. Syphilis crosses the placenta and can infect a baby in the womb, sometimes causing stillbirth or severe birth defects. In 2022 alone, there were an estimated 700,000 cases of congenital syphilis worldwide. Gonorrhea, chlamydia, hepatitis B, and genital herpes can pass to a baby during delivery as the baby moves through the birth canal. HIV can cross the placenta during pregnancy or transmit during birth, though antiviral treatment during pregnancy lowers this risk to less than 1%.
Breastfeeding is another potential route for HIV specifically. People with herpes or syphilis can breastfeed safely as long as the baby and any pumping equipment don’t touch an active sore.
Shared Needles and Blood Contact
HIV, hepatitis B, and hepatitis C spread through blood. Sharing needles for drug injection is the most significant non-sexual transmission route for these infections. Unsterilized tattoo or piercing equipment poses a similar risk. Occupational transmission (such as a healthcare worker getting stuck with a contaminated needle) is extremely rare. Only 58 confirmed cases of occupational HIV transmission have been reported in the entire history of tracking in the United States.
What Doesn’t Spread STIs
Bacterial STIs cannot survive outside the human body for more than a very short time. Some viral STIs like herpes and hepatitis B can technically survive on surfaces briefly, but there is virtually zero chance of catching an STI from a toilet seat, a doorknob, a swimming pool, or shared utensils. These organisms need the warm, moist environment of human tissue to thrive. Casual contact like hugging, shaking hands, or sharing food does not transmit STIs.
How Vaccines Change the Picture
Two STIs now have highly effective vaccines. The HPV vaccine is close to 100% effective at preventing infection with the virus types it targets when given before exposure. The current nine-strain vaccine covers the HPV types responsible for about 81% of cervical cancers in the United States, plus the two types that cause most genital warts. It works best when given before someone becomes sexually active, since it has no effect against HPV types a person has already been infected with.
The hepatitis B vaccine is similarly effective and is routinely given in infancy in most countries. Together, these two vaccines eliminate the risk of some of the most consequential STIs entirely, making vaccination one of the most powerful prevention tools available alongside condom use and regular testing.
Why Many People Spread STIs Without Knowing
A major reason STIs are so common is that many cause no symptoms at all, especially in the early stages. Chlamydia and gonorrhea are frequently “silent” infections, particularly in women. HPV almost never causes noticeable symptoms. Herpes can be mild enough to go unnoticed or be mistaken for an ingrown hair. Syphilis sores are painless and can appear in hard-to-see locations. Because of this, people often transmit infections without ever realizing they have one. Regular screening is the only reliable way to catch these infections early, since waiting for symptoms means many cases go undetected and untreated for months or years.