You get an STD (also called an STI) when bacteria, viruses, or parasites pass from an infected person to you, most commonly during vaginal, anal, or oral sex. But sex isn’t the only route. Some infections spread through skin-to-skin contact, shared needles, or from a mother to her baby during pregnancy or birth.
Sex With an Infected Partner
The most common way STIs spread is through contact with infected bodily fluids: semen, pre-ejaculate, vaginal fluid, or blood. During vaginal or anal sex, these fluids come into direct contact with mucous membranes, the thin, moist tissue lining the genitals, rectum, and mouth. These membranes are far more permeable than regular skin, which is why they’re the primary entry point for infections like chlamydia, gonorrhea, HIV, and hepatitis B.
Penetration doesn’t have to happen for transmission to occur. Infections can be present in pre-ejaculate fluid, and some spread when your genital area simply touches another person’s genitals. This means any genital contact carries some level of risk, even without full intercourse or ejaculation.
Skin-to-Skin Contact
Several STIs don’t need fluids at all. Herpes, HPV (the virus behind genital warts), and syphilis spread through direct skin-to-skin contact with an infected area. If someone has a herpes sore, a syphilis chancre, or HPV-affected skin, touching that area with your own skin or mucous membranes can transmit the infection.
This is an important distinction because condoms, while highly effective at blocking fluid-borne infections like gonorrhea and chlamydia, offer less protection against skin-to-skin infections. A condom only covers part of the genital area, so any exposed skin that touches an infected spot can still pick up herpes, HPV, or syphilis.
Oral Sex
Oral sex is lower risk than vaginal or anal sex, but it’s not risk-free. Chlamydia, gonorrhea, syphilis, herpes, HPV, and HIV can all be transmitted through oral sex. In a study of gay men diagnosed with syphilis, 1 in 5 reported oral sex as their only sexual contact. HIV risk from oral sex is extremely low by comparison, much lower than from vaginal or anal sex.
Cold sores on the mouth (caused by herpes simplex virus type 1) can also spread to a partner’s genitals during oral sex, causing genital herpes. The reverse is true too: genital herpes can pass to the mouth. Oral-anal contact can transmit hepatitis A, hepatitis B, and intestinal parasites.
Shared Needles and Blood Products
Sharing a needle while injecting drugs is one of the most efficient non-sexual ways to get an STI. HIV, hepatitis B, and hepatitis C all spread readily through shared needles because a small amount of infected blood is injected directly into the bloodstream. Blood transfusions were historically a significant transmission route for these same infections, though modern screening has made that risk extremely low in high-income countries.
From Mother to Baby
Certain STIs pass from a pregnant person to their baby during pregnancy, labor, or delivery. Gonorrhea, chlamydia, HIV, and syphilis can all be transmitted this way. Congenital syphilis, where a baby is born with the infection, remains a serious public health concern. This is why STI screening is a routine part of prenatal care. Some infections can be treated with antibiotics safe for use during pregnancy, while others may be managed with antiviral medications to reduce the chance of passing the infection to the baby.
Shared Sex Toys
Using sex toys that haven’t been cleaned between partners can pass infections like chlamydia, syphilis, and herpes. The risk works the same way as direct contact: infected fluids or skin cells on the toy come into contact with another person’s mucous membranes.
Why People Spread STIs Without Knowing
One of the biggest reasons STIs are so common is that many infections produce no symptoms at all. Chlamydia, gonorrhea, HPV, herpes, and HIV can all be present in someone who feels completely fine. A person with no visible sores, no discharge, and no pain can still pass an infection to a partner. This is why relying on symptoms to judge whether someone is “clean” doesn’t work. Regular testing is the only reliable way to know your status.
Each infection also has a window period between exposure and when a test can detect it. Chlamydia and gonorrhea typically show up on tests within one to two weeks. Syphilis takes longer, with blood tests catching most cases by one month and nearly all by three months. HIV can be detected as early as two weeks with a blood-based antigen/antibody test, though an oral swab may take up to three months to catch almost all infections. If you test too soon after exposure, you could get a false negative.
What Doesn’t Give You an STI
Most bacteria and viruses that cause STIs need the specific warmth and moisture of the human body to survive. They die quickly on dry surfaces. You won’t catch an STI from a toilet seat, a towel, or a hot tub. Chlorine and heat kill most of these pathogens in pools and spas. Casual contact like hugging, shaking hands, or sharing food also poses no risk.
Who Faces Higher Risk
Certain factors increase how easily an infection takes hold. Any cuts or broken skin in the genital area make it easier for pathogens to enter the body. Adolescent girls may face higher risk because of a normal developmental feature called cervical ectopy, where the delicate cells of the inner cervix are more exposed than they are in older adults, creating a larger vulnerable surface. Having an existing STI also increases susceptibility to acquiring another one, partly because sores or inflammation compromise the body’s natural barriers.
Anal sex carries higher transmission risk than vaginal sex for most infections because the lining of the rectum is thinner and more easily damaged, allowing pathogens more direct access to the bloodstream.