You can get an STD (also called an STI) through sexual contact, blood exposure, or skin-to-skin touch with an infected person. The specific route depends on the infection: some spread through bodily fluids like semen, vaginal secretions, and blood, while others pass through direct skin contact alone. Here’s how each route works and what actually puts you at risk.
Sexual Contact: The Primary Route
Vaginal, anal, and oral sex are by far the most common ways STDs spread. During these activities, infectious organisms move between partners through semen, vaginal fluids, blood, and pre-ejaculate. Infections like chlamydia, gonorrhea, HIV, and hepatitis B all transmit this way.
Not all sexual activities carry equal risk. For HIV specifically, unprotected vaginal or anal intercourse is classified as high risk. Oral sex without a barrier is lower risk but not zero, particularly when performed on a male partner with ejaculation. Using a condom correctly during vaginal or anal sex drops the risk to the low category, though correct use matters: that means water-based lubricants only and no spillage of semen.
Anal sex tends to carry higher transmission risk than vaginal sex for most infections because the lining of the rectum is thinner and tears more easily, creating direct access to the bloodstream. The receptive partner faces greater risk than the insertive partner in both anal and vaginal sex, though transmission can go in either direction.
Skin-to-Skin Transmission
Some of the most common STDs don’t need fluid exchange at all. Herpes (HSV) and HPV spread through direct contact with infected skin, even when no sores or warts are visible. This is what makes these two infections so widespread.
Herpes simplex virus type 2 (the strain most associated with genital herpes) transmits during sex through contact with genital or anal skin, sores, or fluids. The critical detail: HSV-2 is often transmitted when the skin looks completely normal, with no visible outbreak. HSV-1, the strain usually linked to cold sores, mainly spreads through contact with sores, saliva, or skin around the mouth, but it can also reach the genital area through oral sex.
Condoms reduce herpes risk but don’t eliminate it because the virus can live on genital or anal skin that the condom doesn’t cover. The same applies to HPV, which spreads through skin contact in the genital region and can transmit to areas a condom doesn’t protect.
Pubic lice, scabies, molluscum contagiosum, and mpox can also spread through skin-to-skin contact during sex, though these infections aren’t exclusively sexual. You can pick them up through other forms of close physical contact as well.
Blood and Shared Needles
HIV, hepatitis B, and hepatitis C are the major bloodborne STDs. While sexual contact is one route for all three, they also spread when infected blood enters another person’s body directly. Sharing needles or syringes for drug use is the most significant non-sexual route.
The risk varies dramatically by pathogen. A single needlestick with HIV-containing blood carries roughly a 0.23% chance of transmission, or about 1 in 500. Hepatitis B is far more infectious: the risk from a contaminated needle ranges from 6% to 30%. Hepatitis C falls in between at about 1.8%. Sharing any drug paraphernalia that contacts blood, including pipes and straws, can also pose a risk for hepatitis.
From Mother to Baby
Several STDs can pass from a pregnant or breastfeeding parent to their child, and the timing depends on the infection. Syphilis crosses the placenta and can infect the baby during pregnancy itself. HIV can also cross the placenta or transmit during delivery. Gonorrhea, chlamydia, hepatitis B, and genital herpes typically pass to the baby during birth as the child moves through the birth canal.
Breastfeeding adds another layer of risk for certain infections. With syphilis or herpes, breastfeeding is safe as long as the baby and any pumping equipment don’t touch an active sore. These infections can cause sores anywhere on the breast, including the nipple. Routine prenatal screening catches most of these infections early enough to reduce the risk to the baby significantly through treatment.
What Doesn’t Transmit STDs
Toilet seats, swimming pools, and shared towels come up constantly in conversations about STD risk, but the reality is reassuring. Bacterial STDs like chlamydia and gonorrhea cannot survive outside the human body for more than a very brief period. Some viral STDs, including herpes and hepatitis B, can technically survive on surfaces for a short time, but transmission would require something very specific: fresh infected fluid on a surface making direct contact with an open wound on your body.
In practical terms, you’re unlikely to catch an STD from a toilet seat unless you can see visible liquid or other matter on it. Casual contact like hugging, handshakes, sharing food, or sitting next to someone also poses no risk. STD organisms are adapted to survive in the warm, moist environment of the human body, and they don’t last on dry environmental surfaces.
Testing Windows After Exposure
If you think you’ve been exposed, getting tested too early can produce a false negative because the infection hasn’t built up enough to detect. Each STD has a specific window period you need to wait through before a test becomes reliable.
Chlamydia has the shortest window. A urine test or swab will catch most infections after just one week, and waiting two weeks catches nearly all cases. Syphilis takes longer: a blood test picks up most infections at one month, but you may need to wait three months for the most reliable result. HIV testing depends on the type of test. A blood draw that checks for both antigens and antibodies catches most infections by two weeks and nearly all by six weeks. The oral swab test is less sensitive early on, detecting most cases at one month and nearly all at three months.
If you’re in a situation where you know the exposure was recent, getting a baseline test and then retesting after the appropriate window has passed gives you the most complete picture. Many clinics will walk you through this timing based on what you were potentially exposed to.