BACTERIAL STI / CARE & TRANSMISSION
Treponema pallidum
Syphilis is curable, and the stage guides treatment. Early infection can pass through contact with a sore that is painless, hidden or outside a condom's coverage. Testing when there is a reason to, treatment matched to the stage, and telling the right partners stop it progressing or passing on. [21][22]
Transmission risk
Risk by type of contact
Bottom line. Syphilis spreads through contact with infectious lesions, including oral lesions. Condoms help when they cover the contact area, but uncovered lesions and kissing an oral lesion remain possible routes.
General
Oral
Vaginal
Anal
Kissing
What changes the risk (3)
- Doxy-PEP. HIGH in studied populations: CDC 2024 summarizes greater than 70% lower syphilis incidence in randomized trials. This is not 70% protection for every act or every population. Sources 6
- Stage and treatment. LOW for an exact multiplier: infectious lesions, stage, location and successful treatment materially change transmission; a numeric stage multiplier is not established. Sources 1
- Circumcision. MEDIUM: a Ugandan randomized trial did not establish protection against syphilis (adjusted HR 1.10, 95% CI 0.75-1.65). Do not label circumcision preventive for syphilis. Sources 7
Condoms and dental dams
Lesion coverage provides partial protection; no robust infection-specific condom or dental-dam efficacy percentage across routes. Historical model parameters are not condom studies. Evidence: Low Sources 2, 4
Sources for this risk profile (7)
- CDC CDC, 2021. Syphilis - STI Treatment Guidelines. cdc.gov
- WHO WHO fact sheet, 2025. Syphilis. who.int
- Gray RT et al. Sexually Transmitted Diseases, 2010. Frequent testing of highly sexually active gay men is required to control syphilis. DOI: 10.1097/OLQ.0b013e3181ca3c0a. pubmed.ncbi.nlm.nih.gov
- Stoltey JE, Cohen SE Sexual Health, 2015. Syphilis transmission: a review of the current evidence. DOI: 10.1071/SH14174. pmc.ncbi.nlm.nih.gov
- CDC CDC, 2024. About STI Risk and Oral Sex. cdc.gov
- Bachmann LH et al.; CDC MMWR Recommendations and Reports, 2024. CDC Clinical Guidelines on the Use of Doxycycline Postexposure Prophylaxis for Bacterial Sexually Transmitted Infection Prevention, United States, 2024. DOI: 10.15585/mmwr.rr7302a1. cdc.gov
- Tobian AAR et al. New England Journal of Medicine, 2009. Male circumcision for the prevention of HSV-2 and HPV infections and syphilis. DOI: 10.1056/NEJMoa0802556. pubmed.ncbi.nlm.nih.gov
* How to read these figures. Each is a published estimate under the conditions shown beside it, usually an untreated partner and a single act or partnership. It is not a prediction of your personal risk.
- Evidence: High
- Several prospective studies or a systematic review broadly agree.
- Evidence: Medium
- One or two studies, wide uncertainty, or a model.
- Evidence: Low
- Guideline judgment, case reports, or extrapolation.
Where no risk level is shown, the evidence cannot rank it. That is not the same as low.
Evidence reviewed through 2026-10-27. For reference only; not medical advice.
When to test after a possible exposure
Blood testing may need an early assessment plus repeat testing; a common follow-up pattern is 4-6 weeks and about 12 weeks / 3 months.
When to be seen sooner. New sores, a rash, changes in vision, hearing or nerves, or a confirmed contact are reasons to be seen rather than waiting for a scheduled repeat.
-
Treponemal plus nontreponemal serology
No single first-positive day applies. CDC postassault guidance uses repeats at 4-6 weeks and 3 months; Australian guidance uses a 12-week repeat after recent exposure.
Baseline testing can find an earlier infection. Follow the clinic's follow-up plan; after a confirmed recent contact, a clinic may offer treatment before results are back.
Specimens: Blood
Sources 1, 2, 3
-
Direct lesion testing where available
A lesion can be assessed and sampled when present; blood tests can still be negative in early infection.
Seek assessment promptly for a new genital, anal or oral lesion.
Specimens: Material from a suspicious ulcer / lesion
Sources 1
Early negatives, after treatment, and screening limits
An early negative result. A negative early blood test does not rule out primary syphilis. NHS services differ: some list a 6-week screening interval, others 12 weeks. These are not interchangeable claims of identical assay sensitivity; preserve the local repeat plan.
After treatment. Treatment response is assessed through clinical review and quantitative nontreponemal titers. Treponemal antibodies can persist after successful treatment; remaining antibody-positive is not itself treatment failure.
Screening limits. Prior treated syphilis changes interpretation. A known recent early-syphilis contact can need presumptive treatment even if blood tests are negative.
Kissing and body sites. A new mouth sore after kissing is worth having looked at; there is no separate kissing blood-test window.
Sources for testing guidance (5)
- CDC STI Treatment Guidelines. Syphilis: diagnostic considerations and partner management. 2021. cdc.gov
- CDC STI Treatment Guidelines. Sexual Assault and Abuse and STIs: follow-up testing. 2021. cdc.gov
- Australian STI Management Guidelines. Standard Asymptomatic Checkup. Live guidance. sti.guidelines.org.au
- NHS inform, Scotland. STI testing at home. Live guidance. nhsinform.scot
- Hope House NHS Sexual Health Services. STIs: when to test. Live guidance. hopehouse.nhs.uk
Treatment
Penicillin is the foundation of treatment; the form, route and number of doses depend on the stage. Nerve, eye or ear involvement needs a different regimen and prompt specialist care. In pregnancy, penicillin is what reliably treats the baby too; a reported allergy is evaluated and, when indicated, desensitization is offered. [21][68][69]
Actions
1. Test and assess the stage. Testing usually combines a treponemal test with a quantitative test such as RPR, and an early negative may need repeating. Eye symptoms, changes in hearing, nerve symptoms, or pregnancy after an exposure are reasons to be seen promptly. [21][65][69]
2. Tell partners by stage. CDC windows: primary syphilis, three months plus the time with symptoms; secondary, six months plus the time with symptoms; early latent, one year. A clinic may offer treatment to someone with direct sexual contact in the 90 days before an early-syphilis diagnosis, even if their blood test is negative. [21]
3. Keep track of follow-up. Avoid sexual contact with sores and follow the clinic's advice on when to resume sex. Keep baseline and follow-up RPR results: antibodies can stay positive after cure, so a positive treponemal test on its own does not show reinfection. [21][65][68]
SCIENCE & CONTEXT
Syphilis / the evidence
History
The causative organism was identified in 1905. Effective penicillin treatment was reported in 1943 and remains central to care. Historical descriptions of illness do not definitively establish the organism's geographic origin. Modern testing distinguishes evidence of exposure from markers used to monitor treatment response. [23][65]
Impact
T. pallidum is a spiral-shaped bacterium that can spread beyond where it entered. Primary syphilis often causes a painless sore; secondary syphilis can bring a rash, mouth or genital sores and feeling unwell. Latent infection has no symptoms. Without treatment it can, over time, affect the nervous system, eyes, ears, heart or other organs; treatment at any stage stops it progressing. [21][69]
Late or latent infection is not as sexually infectious as early sores. In pregnancy, infection can pass to the baby, which is why testing in pregnancy matters. Stage, treatment history and repeated results say more than a single positive label. [21][65]
Probability at a glance
| Source state | Sexual transmission interpretation |
|---|---|
Early lesion present | Meaningful risk; oral or uncovered contact can transmit |
Latent / late stage | Do not assign the early-lesion per-act estimate |
Treated; antibody remains positive | Not sufficient evidence of current infectiousness |
Known early exposure within 90 days | Clinical treatment may be indicated despite a negative test |
Stage-specific interpretation is more useful than a generic prevalence calculation. A historical positive antibody result is not equivalent to current infectious syphilis. [21][65]
In short: after a confirmed contact there is no need to wait for a sore. Talking with a clinic early, and their view on treatment, matters more than any per-act percentage.