Short answer

Syphilis transmits through direct contact with infectious lesions, chancres and moist secondary lesions (mucous patches, condyloma lata) encountered during vaginal, anal, or oral sex, plus transplacental passage at any stage.

Infectivity tracks lesions, highest in primary and secondary stages; condoms halve risk without closing it (sores colonize uncovered sites); and blood-test screening catches what barriers miss.

Every element of prevention follows lesion logic.

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Stage infectivity map. When transmission happens

  • Primary: chancre contact transmits highly efficiently (painless firm ulcer teeming with spirochetes. The painlessness is what makes it dangerous, since sex continues).
  • Secondary: mucous patches and condyloma lata (moist genital, anal, and oral lesions) carry heavy organism loads.
  • Early latent: relapsing infectious lesions remain possible within the first year-ish.
  • Late latent and tertiary: sexual transmission rare. But pregnancy transmission persists at every stage, including latent ones.

Recency predicts infectiousness; latency does not equal safety for pregnancies.

Route specifics. Every documented pathway

  • Vaginal, anal, and oral sex with lesion contact (any orifice pairing. Orientation-neutral).
  • Kissing with oral lesions present (chancre, mucous patches. Uncommon but documented; casual kissing without sores carries no syphilis risk).
  • Transfusion (historical. Modern blood screening closed this route in practice).
  • Transplacental passage (devastating and entirely preventable. The congenital syphilis tragedy is a screening failure, not a medical mystery).
  • Needlestick edge cases (occupational exposures with protocols).

The route map in full context lives at How Do Sexually Transmitted Infections Spread.

Why condoms under-protect. Location, location

Chancres colonize lips, fingers (manual contact sites), perineum, anus, and other uncovered skin. Barriers cover the shaft, not the surroundings.

Condoms roughly halve syphilis risk (worth using. Halving matters) without closing it.

Serologic screening of sexually active networks closes the gap condoms leave: blood tests detect syphilis regardless of sore location, which is why syphilis serology belongs in routine panels even for consistent condom users.

  • Prompt lesion evaluation with same-day treatment (penicillin per stage. Infectiousness collapses rapidly on therapy).
  • Stage-based partner look-backs spanning months (syphilis notification windows run longer than bacterial-STI 60-day norms. Clinicians set them by stage).
  • Abstinence through therapy completion (lesions heal, titers confirm).
  • Pregnancy screening triads (first visit, third trimester, delivery for high-risk pregnancies. Each draw catching what earlier ones missed).

When to seek care

For any genital/oral sore or unexplained rash (test before assuming), after partner syphilis diagnoses (stage-matched evaluation), and in pregnancy per screening schedules.