Sexually transmitted infections spread through two basic routes: contact with infected body fluids (blood, semen, vaginal fluid) or direct skin-to-skin contact with an infected area. The specific route depends on the infection, but most transmission happens during vaginal, anal, or oral sex. Some infections can also spread without sex at all.
Fluid-Based vs. Skin-to-Skin Transmission
Not all STIs travel the same way, and understanding the difference helps explain why some are easier to catch than others.
Infections like chlamydia, gonorrhea, and HIV spread primarily through body fluids. During sex, infected semen, vaginal fluid, or blood comes into contact with the mucous membranes lining the genitals, rectum, or mouth. These membranes are thin, moist tissues that absorb pathogens far more easily than regular skin. This is why unprotected vaginal and anal sex carry the highest risk for these infections: there’s direct, sustained contact between fluids and mucous membranes.
Other infections, including herpes (HSV), HPV, and syphilis, spread through skin-to-skin contact. The virus or bacteria lives in or on the skin itself, so touching an infected area is enough. You don’t need a visible sore to be contagious. For herpes, most people who pass the virus to a partner don’t even know they’re infected. Studies have found that people with genital herpes shed the virus on about 3% of days when they have no visible lesions at all.
Risk During Different Types of Sex
Vaginal and anal sex are the highest-risk activities for most STIs. Anal sex carries particular risk because the lining of the rectum is thinner and more prone to small tears, which creates easy entry points for pathogens. For chlamydia specifically, researchers estimate a transmission probability of roughly 4.5% per single act of unprotected sex.
Oral sex is lower risk but far from zero risk. Chlamydia, gonorrhea, syphilis, herpes, HPV, and HIV can all be transmitted through oral sex. You can get an infection in your throat from performing oral sex on someone with a genital infection, and you can get a genital infection from receiving oral sex from someone with an infection in their mouth or throat. The CDC notes that HIV risk from oral sex is much lower than from vaginal or anal sex, but that may not be true for other infections, particularly gonorrhea and herpes.
Oral-anal contact can transmit hepatitis A, hepatitis B, and intestinal infections including parasites and bacteria like E. coli and Shigella.
Why People Catch STIs Without Knowing
One of the trickiest things about STIs is that the person passing them along often has no idea they’re infected. Many STIs produce mild symptoms or none at all. Chlamydia is notoriously silent, especially in women. Gonorrhea can be asymptomatic in the throat or rectum. And herpes spreads largely through asymptomatic shedding, meaning the virus is active on the skin surface even when there are no sores. Most people who acquire herpes get it from a partner who has never been diagnosed.
This is why “they looked healthy” or “they said they were clean” isn’t reliable protection. Someone can test negative on a routine screening and still carry an infection that wasn’t included in the panel, since standard STI testing doesn’t automatically cover herpes or HPV.
Non-Sexual Ways STIs Spread
Several infections commonly grouped under the STI label can spread without any sexual contact. The common thread is exposure to infected blood or body fluids. Specific routes include sharing needles or syringes, getting a tattoo or piercing with improperly sterilized equipment, sharing sex toys that haven’t been cleaned between users, and, rarely, receiving a contaminated blood transfusion.
Mother-to-child transmission is another significant route. An untreated syphilis infection during pregnancy can be devastating: roughly 40% of babies born to mothers with untreated syphilis are stillborn or die shortly after birth. For hepatitis B, up to 90% of infants born to infected mothers will become infected themselves if they don’t receive preventive treatment right after birth. HIV can also pass from mother to child during pregnancy, delivery, or breastfeeding, though modern treatment has made this far less common.
What Condoms Protect Against (and What They Don’t)
Condoms are highly effective against infections that travel in genital fluids. When used consistently and correctly, latex or polyurethane condoms significantly reduce the risk of catching gonorrhea, chlamydia, trichomoniasis, and HIV. These infections need fluid-to-membrane contact, and a condom blocks that exchange.
Protection drops for infections that spread through skin contact. Herpes, HPV, and syphilis can all live on skin that a condom doesn’t cover, like the base of the penis, the scrotum, the inner thighs, or the vulva. Condoms still reduce the risk of these infections, but the CDC describes that protection as “limited” compared to the near-complete barrier they provide for fluid-borne STIs. Dental dams offer similar partial protection during oral sex by blocking direct contact between the mouth and genital or anal skin.
Factors That Increase Your Risk
Certain situations make transmission more likely. Having multiple partners increases the statistical odds of encountering someone who’s infected. Using drugs or alcohol before sex makes it harder to use protection consistently. Having an existing STI, especially one that causes sores or inflammation, makes it easier to catch a second infection because the skin barrier is already compromised.
Biology plays a role too. Receptive partners (the person being penetrated) generally face higher risk than insertive partners because their mucous membranes are more exposed. Younger women may be more susceptible to chlamydia and gonorrhea because the cells at the opening of the cervix are more exposed during adolescence and early adulthood.
Anal sex without a condom is one of the highest-risk activities for HIV transmission specifically, because the rectal lining tears easily and is rich in the types of immune cells HIV targets. Vaginal sex carries meaningful but lower per-act risk, and oral sex sits at the lower end of the spectrum for HIV, though it remains relevant for bacterial infections and herpes.
How to Lower Your Risk
Using condoms or dental dams every time reduces risk across the board, with stronger protection for fluid-borne infections and partial protection for skin-contact infections. Getting tested regularly, and asking partners to do the same, catches infections early and breaks the chain of unknowing transmission. If you’re at higher risk for HIV, pre-exposure prophylaxis (PrEP) reduces the chance of catching it by over 99% when taken as directed.
Vaccination is available for two STIs. The HPV vaccine prevents the strains responsible for most genital warts and HPV-related cancers. Hepatitis B vaccination is routine in childhood but is also available for unvaccinated adults. Both are highly effective at preventing infection entirely, which no barrier method can guarantee.