Sexually transmitted infections (STIs) spread when bacteria, viruses, or parasites pass from one person to another, most commonly through vaginal, anal, or oral sex. The pathogens enter your body through mucous membranes or tiny, invisible tears in the skin that occur naturally during sexual contact. More than 2.2 million STI cases were reported in the United States in 2024 alone, and the vast majority of people who pass an infection to someone else don’t know they’re infected. Roughly 80% of new bacterial STI cases involve a source partner with zero symptoms.
How Pathogens Get Inside Your Body
Your skin is an effective barrier against most infections, but mucous membranes are not. The lining of the vagina, the urethra inside the penis, the rectum, the mouth, and the throat are all thin, moist tissues that STI-causing organisms can penetrate easily. During sex, microscopic abrasions form on these surfaces even when everything feels normal. Those tiny breaks give bacteria and viruses a direct path into your bloodstream or underlying tissue.
This is why sexual contact is such an efficient route of transmission. It involves sustained, direct contact between two people’s mucous membranes and body fluids (semen, vaginal fluid, rectal secretions, and sometimes blood) at the same time that micro-tears are forming. You don’t need a visible cut or sore for transmission to happen.
Fluid-Based vs. Skin-to-Skin Infections
Not all STIs spread the same way, and understanding the difference explains why some are harder to prevent than others.
Some infections travel primarily through body fluids. HIV, chlamydia, gonorrhea, and hepatitis B require semen, vaginal fluid, rectal secretions, or blood to reach a new host’s mucous membranes. For these infections, any barrier that blocks fluid exchange significantly lowers risk.
Others spread through direct skin-to-skin contact. Herpes (HSV) and HPV fall into this category. Herpes transmits when the virus on one person’s skin or in their saliva contacts another person’s mucous membrane or abraded skin. HPV works similarly, spreading through contact with infected skin in the genital area. Because these viruses live on skin surfaces that a condom doesn’t fully cover, they can transmit even during protected sex. Syphilis also spreads through direct contact with a sore, which can appear on areas a condom doesn’t reach.
Oral Sex Carries Real Risk
A common misconception is that oral sex is safe from STI transmission. It isn’t. Chlamydia, gonorrhea, syphilis, herpes, HPV, and HIV can all pass through oral contact with a partner’s genitals. In a study of men with syphilis, 1 in 5 reported that oral sex was their only sexual contact. The risk of HIV transmission through oral sex is substantially lower than through vaginal or anal sex, but that lower risk doesn’t extend to other infections.
Oral contact with the anus can also transmit hepatitis A and B, along with intestinal parasites and bacteria like E. coli and Shigella. These aren’t traditionally classified as STIs, but they spread readily through this type of sexual contact.
Why Anal Sex Carries Higher Risk
The rectal lining is thinner and more fragile than vaginal tissue, which means it tears more easily during penetration. Those tears create larger entry points for pathogens. The rectum also has a rich blood supply close to the surface, giving infections like HIV a more direct route into the bloodstream. This is why receptive anal sex carries the highest per-act risk of HIV transmission of any sexual activity.
Non-Sexual Routes of Transmission
Sex is the primary route, but it’s not the only one. Sharing needles or syringes is a direct transmission path for HIV and hepatitis B and C. Any situation where one person’s blood enters another person’s body creates potential for these blood-borne infections.
A pregnant person can also pass certain STIs to their baby during pregnancy, labor, or breastfeeding. Herpes transmitted during birth can cause brain infection, organ damage, or death in a newborn, with the highest risk coming from a first outbreak late in pregnancy. Syphilis can cross the placenta and infect a developing fetus, a condition called congenital syphilis, which rose 82% in the U.S. over five years to nearly 4,000 cases in 2024. HIV can pass to a baby during pregnancy or breastfeeding, though early treatment during pregnancy dramatically reduces this risk. Even HPV can occasionally transfer from mother to child, in rare cases causing growths on the baby’s voice box.
Why You Can’t Tell Who Has an STI
The single biggest factor driving STI transmission is that most infected people look and feel completely fine. Research on bacterial STIs found that all existing infections at the start of the study and 80% of newly acquired infections produced no symptoms at all. Someone with chlamydia, gonorrhea, or early syphilis often has no pain, no discharge, and no visible sores. Herpes can shed from the skin and transmit to a partner even between outbreaks, when no blisters are present.
This means you cannot assess risk by how a partner looks, how they feel, or even by asking if they’ve been tested recently (since many people haven’t been tested for everything, or were tested before their most recent exposure). The absence of symptoms is the norm, not the exception.
How Much Protection Condoms Actually Provide
Condoms work, but their effectiveness depends heavily on whether they’re used correctly every time. The largest study designed specifically to measure condom effectiveness against bacterial STIs found that simply using condoms consistently (putting one on every time) did not produce a statistically significant reduction in infections. But when people used condoms both consistently and correctly, their odds of acquiring an infection dropped by 59%.
That gap matters. “Correct” use means putting the condom on before any genital contact, using it the entire time, and removing it properly. Many people use condoms inconsistently, applying them partway through sex or using them only some of the time, which sharply reduces the protection.
For skin-to-skin infections like herpes and HPV, condoms provide partial protection because they don’t cover all potentially infected skin. HPV vaccines offer strong protection against the strains responsible for most genital warts and HPV-related cancers, making vaccination one of the most effective prevention tools available for that specific infection.
The Most Common STIs by the Numbers
Chlamydia is by far the most frequently reported STI in the U.S., with roughly 1.5 million cases in 2024. Gonorrhea came next at about 543,000 cases, followed by syphilis at 190,000. These three infections are all caused by bacteria and are curable with antibiotics when caught. The challenge is catching them, since most cases are asymptomatic and go undiagnosed without routine screening.
Viral STIs like herpes and HPV are even more common in the general population but aren’t tracked through the same reporting system. HPV is so widespread that nearly all sexually active people will contract at least one strain at some point in their lives, though most clear the virus without ever knowing they had it.