Short answer
STIs travel by sex (vaginal, anal, oral), skin-to-skin genital contact, blood, and from parent to child around birth. And never by casual contact (toilets, pools, hugging, food, mosquitoes, casual kissing).
Knowing which route serves which infection turns vague fear into precise prevention: fluid routes need condoms and testing, skin routes add vaccines and visual checks, blood routes need sterile equipment, perinatal routes need pregnancy screening.
The route map below covers every infection’s pathways.
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See Tests & Prices →Sexual routes. The main highways
- Vaginal and anal sex transmit the full range (chlamydia, gonorrhea, trichomoniasis, HIV, syphilis, herpes, HPV, hepatitis B. Anal carrying the highest per-act HIV risk).
- Oral sex transmits gonorrhea, chlamydia, syphilis, and herpes in both directions with negligible HIV risk outside blood exposure.
- Genital contact without penetration still spreads skin-to-skin infections (HPV, herpes, syphilis, molluscum. “no penetration” reduces fluid-borne risk but never zeroes STI risk).
Route-specific detail: anal contexts at Can You Get an STI from Anal Sex, oral contexts at Can You Get an STI from Oral Sex.
Blood routes. Efficient, narrow, preventable
- Sharing injection equipment (needles, syringes, cookers, cotton, rinse water) transmits HIV and hepatitis B/C efficiently through direct blood exchange. The dominant blood-borne route.
- Needlesticks and blood-contaminated sharps (occupational exposures with PEP protocols).
- Transfusions (now extremely safe in the US blood supply through donor screening plus nucleic-acid testing of every unit. Effectively historical risk).
Never share any injection item; syringe service programs supply sterile equipment confidentially.
Perinatal routes. Preventable with screening
HIV, syphilis, herpes, hepatitis B, chlamydia, and gonorrhea can pass during pregnancy, delivery, or breastfeeding. And prenatal screening plus timely treatment or prophylaxis prevents nearly all of it.
First-visit panels, third-trimester repeats, and delivery protocols form the safety chain; congenital syphilis cases represent cascade failures of this chain, not medical mysteries.
Report symptoms promptly between scheduled screens rather than waiting for the next visit.
Non-routes that worry people needlessly. Retired permanently
Toilet seats, swimming pools, hugging, coughing, shared food, mosquitoes, and casual kissing transmit no STIs. Organisms die on surfaces within minutes and need mucosal deposition, blood entry, or birth passage.
Energy spent fearing these is better spent on testing, condoms, and vaccines. The interventions addressing real routes.
Worry redirected is prevention funded.
When to seek care
After any sexual or blood exposure, during pregnancy per screening schedules, and for baseline testing when routes or risks are unclear.