Sexually transmitted infections spread when bacteria, viruses, or parasites pass from one person to another, almost always through sexual contact. The transfer happens through two main routes: infected body fluids (semen, vaginal fluid, blood) or direct skin-to-skin contact with an infected area. Understanding which route applies to which infection helps explain why some are easier to prevent than others.
How Infections Enter Your Body
The lining of your genitals, mouth, and rectum is made of soft, moist tissue called mucous membrane. Unlike the tougher skin on your arm or leg, mucous membranes are thin and permeable, which makes them the primary entry point for sexually transmitted pathogens. During sex, tiny abrasions form in this tissue that are invisible to the naked eye. These micro-tears give bacteria and viruses a direct path into your bloodstream or deeper tissue layers.
Anal tissue is especially vulnerable because it’s thinner and more prone to tearing than vaginal tissue. One study estimated the risk of acquiring HIV from a single act of unprotected receptive anal intercourse at about 3.4%, compared to less than 0.01% per act of vaginal intercourse. That 300-fold difference comes down almost entirely to how easily the tissue is damaged and how much blood supply sits just beneath the surface.
Seminal fluid can also play an active role beyond just carrying a pathogen. Research published in Frontiers in Immunology found that seminal plasma can weaken the tight junctions between cells in rectal and genital tissue, increasing permeability and drawing immune cells toward the surface, which paradoxically gives viruses like HIV more target cells to infect.
Fluid-Based vs. Skin-to-Skin Infections
Not all STIs spread the same way, and the distinction matters for understanding your actual risk.
Fluid-based infections require the exchange of body fluids: semen, vaginal secretions, or blood. These include chlamydia, gonorrhea, HIV, and trichomoniasis. Trichomoniasis, caused by a single-celled parasite, directly damages the surface layer of genital tissue and creates tiny ulcerations in the vagina, cervix, or urethra, which can also make you more susceptible to other infections. For this group of STIs, a barrier like a condom is highly effective because it physically blocks the fluid exchange.
Skin-to-skin infections spread through direct contact with infected skin or sores, even in areas a condom doesn’t cover. This group includes genital herpes (HSV-1 and HSV-2), HPV (human papillomavirus), and syphilis. Syphilis, for example, spreads when you touch the painless sore (called a chancre) that appears at the infection site. HPV replicates in the base layer of skin cells and can be present across a broad area of genital skin. Because these infections live on or near the skin surface rather than exclusively in fluids, condoms reduce the risk but can’t eliminate it.
Why Someone With No Symptoms Can Still Transmit
This is one of the most important and least intuitive facts about STIs: the person who passes an infection to you will often look and feel completely healthy. Most herpes transmission, for instance, happens when the infected partner has no visible sores at all.
A study published in JAMA tracked viral shedding in people with HSV-2 and found that even those who had never had a recognized outbreak were shedding the virus on about 10% of days tested. Among people with a history of outbreaks, the virus was detectable on 20% of days. The critical finding was that roughly 84% of shedding days in people without symptoms were “subclinical,” meaning the virus was actively present on the skin surface with no visible sore. The amount of virus shed during these silent episodes was essentially the same as during recognized outbreaks.
Chlamydia and gonorrhea follow a similar pattern. Many people, especially women, carry these bacterial infections with zero symptoms for weeks or months. HPV is almost entirely asymptomatic in most carriers. This is why STIs continue to spread so effectively: the absence of symptoms doesn’t mean the absence of infection.
Condoms Lower Risk, but Technique Matters
Using a condom consistently is a good start, but the protection you actually get depends heavily on whether you’re also using it correctly. A large prospective study tracking daily sexual encounters found that people who used condoms consistently but not always correctly had no statistically significant reduction in STI risk. However, those who used condoms both consistently and correctly had about 59% lower odds of acquiring chlamydia, gonorrhea, or trichomoniasis compared to those who didn’t use condoms at all.
That gap highlights that common mistakes (putting the condom on partway through, using the wrong lubricant, or not leaving space at the tip) meaningfully reduce the protection you’d otherwise get. For skin-to-skin infections like herpes and HPV, condoms provide partial protection because they only cover part of the genital area where the virus may be present.
Non-Sexual Ways STIs Can Spread
While sex is by far the dominant route, a handful of STIs can spread without it. HIV and hepatitis B can transmit through shared needles or other equipment that breaks the skin and allows blood-to-blood contact. Hepatitis B is especially resilient: the virus remains infectious in dried blood on surfaces for at least a week at room temperature.
A pregnant person can also pass certain infections to their baby during pregnancy or delivery. This is called vertical transmission, and it’s a known risk with syphilis, HIV, herpes, gonorrhea, and chlamydia. Herpes poses the greatest danger during delivery if the mother is having an active outbreak, which is why providers monitor for this closely in late pregnancy.
Transmission from surfaces or objects is theoretically possible for a few pathogens but practically very rare. Gonorrhea bacteria can survive on plastic or bed sheets for about 24 hours, though in sharply declining numbers. HPV can persist on surfaces for up to 7 days in laboratory conditions. Chlamydia, by contrast, dies on plastic surfaces within 30 to 120 minutes. These survival times come from controlled lab settings. In real-world conditions, nonsexual transmission of STIs is considered an infrequent event.
Where These Infections Came From Originally
If you’re wondering how STIs entered the human population in the first place, the answer for several major infections is cross-species transmission. HIV is the best-documented example. HIV-1, the strain responsible for the global pandemic, originated from a simian immunodeficiency virus carried by chimpanzees in southeastern Cameroon. Chimpanzees themselves acquired their version of the virus by hunting and eating smaller monkeys, specifically red-capped mangabeys and other species that carried their own strains. The chimpanzee virus became a genetic mosaic of these monkey viruses through recombination.
The jump to humans happened roughly a century ago, most likely when hunters butchering chimpanzees for bushmeat were exposed to infected blood through cuts or mucous membranes. HIV-2, a less common and less aggressive strain found mainly in West Africa, followed a similar path but came from sooty mangabey monkeys rather than chimpanzees. Once these viruses made the leap into humans, sexual transmission became their primary means of spreading through the population.
HPV has been with humans for so long that different strains have co-evolved with our species over hundreds of thousands of years. Herpes simplex viruses similarly trace back to our primate ancestors. Syphilis has a more debated history, but the bacterium that causes it belongs to a family of closely related organisms found in both humans and animals across multiple continents.