How Do You Get an STD? Every Way They Spread

Route 1: infected fluids

Semen, pre-ejaculate, vaginal secretions, rectal fluids and blood all carry pathogens efficiently. Any act moving these fluids between bodies transmits whatever the source carries:

• Vaginal sex: transmits essentially everything — chlamydia, gonorrhea, syphilis, HIV, herpes, HPV, trich, hepatitis B.
• Anal sex: highest per-act efficiency for several pathogens; rectal tissue tears easily.
• Oral sex: passes gonorrhea, syphilis, herpes and HPV readily; HIV less efficiently (per-act numbers).
• Shared injection equipment: blood-to-blood — HIV and hepatitis's fastest highway.

Route 2: skin-to-skin (the condom gap)

Herpes, HPV, molluscum, scabies and pubic lice live on skin itself — not just in fluids. Contact with uncovered skin transmits them, which is why condoms reduce but don't eliminate these infections: they shield what they cover only.

Kissing joins this category for HSV-1 — deep kissing transmits oral herpes reliably enough to make it the planet's most common STI pathway.

Routes 3 & 4: birth and blood

• Mother-to-child: syphilis crosses the placenta; HIV, hepatitis B and herpes transmit during delivery — all screenable in pregnancy, all preventable with treatment.
• Blood exposure: needle sharing, contaminated tattoo equipment (rare in licensed shops), occupational needlesticks.

Risk math for specific scenarios lives throughout our transmission section — start with one-time-exposure odds.

Per-Act Risk Is Not One Number

Transmission probability varies by pathogen, sexual act, and direction of exposure. According to CDC, HIV per-act risk ranges from about 0.04% for insertive penile-vaginal sex to about 1.38% for receptive anal sex.

These are population averages, not individual predictions. Per CDC, ulcerative STIs, high HIV viral load, and other factors can raise transmission risk. Precise per-act estimates for gonorrhea and chlamydia are less consistently quantified because asymptomatic infection makes exposure timing difficult to measure.

Infectious Before Symptoms: Asymptomatic Shedding

An STD can be transmitted by a person who has no symptoms and no visible lesions. According to CDC, most genital herpes infections are transmitted by people who do not know they have the virus, and transmission can occur during asymptomatic viral shedding.

According to WHO, acute HIV infection is associated with a very high viral load and increased transmission risk before symptoms are reliably recognized. Chlamydia is often asymptomatic, and gonorrhea may also be asymptomatic, especially in women and in the throat. Primary syphilis sores may be painless and hidden, and HPV can spread from normal-appearing skin.

This means symptom checks and partner history are weak prevention tools; transmission does not require visible disease.

Vertical Transmission: Timing Changes the Risk

Vertical transmission from mother to child is not a single route; it can occur in utero, during labor and delivery, or through breastfeeding, and each infection has a different critical window.

According to WHO, without intervention, HIV transmission from mother to child ranges from 15% to 45%; with effective antiretroviral treatment and viral suppression, the risk falls below 2%. Syphilis can cross the placenta during pregnancy and cause stillbirth or congenital infection, which is why CDC recommends syphilis screening during pregnancy. Herpes simplex virus is most often transmitted to the newborn during vaginal delivery, especially when primary infection occurs near delivery. Gonorrhea and chlamydia can infect the baby’s eyes or respiratory tract during passage through the birth canal.

These timing-specific risks mean prevention differs by pathogen, not just by a single prenatal test.