Sexually transmitted infections spread when a person is exposed to an infectious pathogen through a route that allows it to establish infection. Sexual transmission can occur through vaginal, anal, or oral sex and through direct genital or skin-to-skin contact, depending on the pathogen. Transmission does not necessarily require ejaculation, and some infections can spread during contact involving infected skin or mucous membranes. The precise route varies considerably between organisms.
HIV requires exposure to certain infectious body fluids, whereas herpes and HPV can spread through direct contact with infected skin. Chlamydia and gonorrhoea can infect genital, rectal, and pharyngeal tissues following relevant sexual exposure. Because pathogens differ in where they are present and how they enter the body, no single description of sexual transmission applies to every STI.
Vaginal intercourse can transmit infections when infected genital secretions or mucosal surfaces come into contact with susceptible tissues. Anal intercourse can also transmit infections and may carry particularly significant risk for certain pathogens because the rectal lining can be vulnerable to microscopic trauma. Oral sex can transmit infections to the mouth or throat and can transmit some infections from the mouth or throat to genital tissues.
Gonorrhoea, syphilis, herpes, HPV, and other infections can be transmitted through oral sexual contact, while HIV transmission through oral sex is substantially less efficient than through vaginal or anal exposure but is not considered biologically impossible under all circumstances. Sexual transmission is therefore not limited to penile-vaginal intercourse, and a sexual health assessment should consider the specific anatomical sites exposed.
Skin-to-skin transmission is particularly important for infections such as herpes simplex virus and HPV. These pathogens can be present on areas of skin that are not covered by a condom, so condoms reduce but cannot completely eliminate transmission risk. Syphilis can also spread when an infectious lesion contacts another person's mucous membrane or damaged skin. This explains why some STIs can be transmitted despite condom use and why visible absence of sores does not guarantee absence of infection. Viral shedding can occur without obvious lesions in herpes infection, and HPV is often asymptomatic. Consequently, the appearance of the genitals is not a reliable diagnostic method for determining whether someone has an STI.
Blood exposure is another major route for certain infections. HIV, hepatitis B, and hepatitis C can be transmitted when infected blood enters another person's bloodstream, most notably through sharing needles or other injecting equipment. Blood-contaminated instruments can transmit infection if infection-control procedures are inadequate, although modern regulated healthcare systems use sterilisation and screening procedures designed to prevent this route. Sharing injecting equipment remains an important preventable transmission mechanism.
Sexual transmission of hepatitis B and HIV is also possible because these pathogens can occur in relevant genital fluids, while hepatitis C is transmitted sexually less efficiently but can be transmitted under circumstances involving blood exposure. The risk therefore depends on both the pathogen and the nature of the exposure.
Some STIs can be transmitted from a pregnant person to a baby during pregnancy, childbirth, or breastfeeding, depending on the infection. HIV can be transmitted vertically during pregnancy, delivery, or breastfeeding, although effective antiretroviral treatment and appropriate medical management can reduce the risk dramatically. Syphilis can cross the placenta and cause congenital syphilis, while gonorrhoea and chlamydia can infect newborns during delivery. Hepatitis B can be transmitted around birth, which is why preventive vaccination and, where indicated, additional measures are important for babies born to infected mothers. These routes are not sexual transmission between adults but are important pathways by which sexually associated infections can reach another person.
Transmission depends on infectiousness as well as exposure. A pathogen must generally be present at a sufficient concentration in an appropriate biological material or tissue and reach a susceptible site. HIV illustrates this clearly: transmission requires certain body fluids, including blood, semen, vaginal fluids, rectal fluids, or breast milk, to enter the bloodstream or contact susceptible mucous membranes or damaged tissue. HIV is not spread through casual contact, hugging, ordinary kissing, sharing utensils, toilet seats, swimming pools, or insect bites. Similarly, an STI diagnosis cannot be inferred from ordinary social contact. Understanding the biological route prevents both underestimation of genuine risk and unnecessary fear of interactions that cannot transmit the infection.
Asymptomatic infection is a major reason STIs continue to spread. A person may have an infection without pain, discharge, sores, fever, or any other obvious sign and can nevertheless transmit it. Symptoms can also disappear while the infection remains present, as can happen with some viral infections or temporarily after partial treatment. Testing is therefore more reliable than symptom observation for identifying many STIs.
The timing of testing matters because the pathogen or immune response may not yet be detectable immediately after exposure. Sexual partners can also transmit an infection without knowing that they have it. Regular screening and prompt treatment can therefore interrupt transmission chains more effectively than relying on symptoms alone.
STI transmission can be reduced by combining prevention measures suited to the specific pathogen and exposure. Condoms reduce the transmission of many infections when used correctly and consistently, but they are less protective against infections spread by uncovered skin. Vaccination provides strong prevention for vaccine-preventable infections such as HPV and hepatitis B.
HIV transmission can be prevented through effective antiretroviral treatment, pre-exposure prophylaxis for people at risk, post-exposure prophylaxis when appropriate after a significant recent exposure, and avoiding shared injecting equipment. Testing and treatment reduce the period during which many bacterial infections can be transmitted. Understanding how a particular STI spreads is therefore essential because prevention should be matched to the biological transmission route rather than assuming that one method provides universal protection.
