Compare published per-act transmission risk and prevalence for chlamydia, gonorrhea, syphilis, HIV, hepatitis B&C, HPV, HSV-2 and more across 21 countries — vaginal, anal and oral sex, with and without a condom. Based on WHO, ECDC and CDC data. An educational reference, not a personal diagnosis.
Partners since 30 days before your last negative test
Partners since 30 days before your last negative test
Risk per single act— compounds with repetition. Tap or hover any value for the breakdown.
Figures are estimated cases per 1,000,000 (e.g. 50,000 ≈ 5%).
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| Activity | ChlamydiaCurable | GonorrheaSerious | SyphilisSerious | HIVLife-threatening | Hepatitis CSerious |
|---|---|---|---|---|---|
Vaginal sex: you are insertive Penis-in-vagina intercourse. | 2,476 | 985 | 154 | 1 | <1 |
Anal sex: you penetrate Penis-in-anus intercourse. | 2,476 | 50 | 154 | 3 | <1 |
Blowjob (fellatio): you receive Oral sex on a penis. | no data | 225 | 154 | <1 | negligible |
Cunnilingus: you give Oral sex on a vulva. | no data | no data | 154 | negligible | negligible |
Grinding Unclothed genital-to-genital contact without penetration. | negligible | negligible | 154 | negligible | negligible |
Fingering Manual stimulation of genitals or anus. | negligible | negligible | negligible | negligible | negligible |
Pregnancy from one act of protected vaginal sex (condom, typical use)
per encounter · Cycle-average risk with typical-use condom effectiveness (~87%) applied.(62)Pregnancy from one act of unprotected vaginal sex
per encounter · Averaged across the menstrual cycle — far higher mid-cycle, near zero outside the fertile window.(62)Catching the flu (symptomatic influenza)
per season · US seasonal average; ranges from 3% to 11% between seasons.(46)Physical or sexual violence by an intimate partner (women aged 15 to 49, worldwide)
lifetime · WHO global estimate among women who have ever been in a relationship.(47): risk per encounter of the susceptible person contracting the STI.
: per-act transmission rate (assuming the infectious partner has the STI), from peer-reviewed meta-analyses.
: probability the partner is infected. By default this is the country prevalence of the STI in the partner's country of residence; personal history can replace it entirely (below).
: male-male (MSM) multiplier. for mixed-sex pairings; when both partners are male it is the STI-specific factor in the table below. (When personal history is used, the MSM adjustment is folded into and .)
1. Male-male contact (MSM). Country prevalence figures are general-population averages and substantially underestimate risk for men who have sex with men. When both participants are male, is multiplied by an STI-specific factor drawn from UNAIDS / ECDC surveillance:
| Chlamydia | Gonorrhea | Syphilis | HIV | Hepatitis B | Hepatitis C | HPV | Herpes (HSV-2) | M. genitalium | Trichomoniasis |
|---|---|---|---|---|---|---|---|---|---|
| ×4 | ×12 | ×20 | ×23 | ×4 | ×3 | ×4 | ×3 | ×3 | ×1 |
2. Personal history. When you enter a partner's recent negative test and their encounter history, the country prevalence is replaced entirely by a personalised estimate , combining a Bayesian posterior after the negative test with the cumulative acquisition risk from each listed encounter:
: prior (country prevalence); when no negative test is entered.
: test sensitivity and specificity for that STI.
: prevalence of the STI for encounter 's partner (with the MSM factor applied when both are male), its per-act transmission rate, and the number of acts.
Because the MSM factor is already inside , this override takes precedence over the standalone term above.
Vaccination. There is no vaccination term in the formula. For the vaccine-preventable STIs (Hep B and HPV), if the susceptible person is marked as vaccinated their cell is simply shown as N/A. This is a simplification: real-world vaccines are highly but not 100% protective (and the HPV vaccine covers only the high-risk types), so treat a vaccinated N/A as "very low", not "zero".
| STI | Prevalence (per 1,000,000) | Notes |
|---|---|---|
| Chlamydia | 20,000 to 34,000(59) | WHO publishes no country-level estimate for this infection, so the WHO European Region figure is shown. A national figure may differ substantially. WHO European Region, 2020 — men 2%, women 3.4%. |
| Gonorrhea | 2,000 to 3,000(59) | WHO publishes no country-level estimate for this infection, so the WHO European Region figure is shown. A national figure may differ substantially. WHO European Region, 2020 — men 0.2%, women 0.3%. |
| Syphilis | 190 to 760(59) | WHO publishes no country-level estimate for this infection, so the WHO European Region figure is shown. A national figure may differ substantially. WHO European Region, 2022 — men 0.08%, women 0.02%. |
| HIV | 1,000 to 2,000(4) | The Netherlands, 2023. |
| Hepatitis B | 3,900 to 5,000(8) | The Netherlands, 2020 — HBsAg prevalence, all ages (uncertainty interval). |
| Hepatitis C | 0 to 1,000(9) | The Netherlands, 2022 — reported as 0.0%, below the reporting precision, so shown as under 0.1%. |
| HPV | 117,000 to 214,000(59) | WHO publishes no country-level estimate for this infection, so the WHO European Region figure is shown. A national figure may differ substantially. Between the global average and the eastern European estimate; western Europe sits below the low end. |
| Herpes (HSV-2) | 53,000 to 107,000(59) | WHO publishes no country-level estimate for this infection, so the WHO European Region figure is shown. A national figure may differ substantially. WHO European Region — men 5.3%, women 10.7%. |
| M. genitalium | 10,000 to 18,000(59) | WHO publishes no country-level estimate for this infection, so the WHO European Region figure is shown. A national figure may differ substantially. Higher-HDI pooled estimate, 1.3% (95% CI 1.0–1.8). |
| Trichomoniasis | 2,000 to 16,000(59) | WHO publishes no country-level estimate for this infection, so the WHO European Region figure is shown. A national figure may differ substantially. Men 0.2%, women 1.6%. |
| Activity | Direction | Chlamydia | Gonorrhea | Syphilis | HIV | Hepatitis B | Hepatitis C | HPV | Herpes (HSV-2) | M. genitalium | Trichomoniasis |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Vaginal | Female → Male | 13,000 to 138,000(49) | 190,000 to 530,000(17) | 30,000 to 397,000(53) | 100 to 1,400(15) | no data(35) | 1 to 10(24) | 3,200 to 27,100(54) | 1,700(25) | 13,000 to 138,000(55) | 13,000 to 138,000(57) |
| Anal | Receptive → Insertive | 13,000 to 138,000(49) | 20,000(50) | 30,000 to 397,000(53) | 400 to 2,800(15) | no data(35) | 1 to 10(52) | 3,200 to 27,100(54) | 1,700(56) | 13,000 to 138,000(55) | no data(30) |
| Blowjob | Receiver of blowjob at risk | no data(32) | 90,000(50) | 30,000 to 397,000(53) | 0 to 400(51) | no data(36) | negligible(24) | no data(54) | no data(56) | no data(31) | negligible(30) |
| Cunnilingus | Giver of cunnilingus at risk | no data(32) | no data(32) | 30,000 to 397,000(53) | negligible(51) | no data(36) | negligible(24) | no data(54) | no data(56) | no data(31) | negligible(30) |
| Grinding | Skin contact | negligible(32) | negligible(32) | 30,000 to 397,000(53) | negligible(15) | no data(36) | negligible(24) | no data(54) | no data(56) | negligible(31) | negligible(30) |
| Fingering | Manual contact | negligible(32) | negligible(32) | negligible(29) | negligible(15) | no data(36) | negligible(24) | negligible(54) | negligible(56) | negligible(31) | negligible(30) |
Each reference gives the authors, publication, year and the exact page, table or section where the number appears, so you can open the source and check it. Entries marked Derived are numbers no source publishes directly — they were calculated here, and each one states the arithmetic and the sources it used.
Where to look: Abstract, “Findings”, p. 548 — 2016 prevalence in adults 15–49: chlamydia 3.8% (women) / 2.7% (men); gonorrhoea 0.9% / 0.7%; trichomoniasis 5.3% / 0.6%; syphilis 0.5% / 0.5%
Used for: Global adult prevalence of chlamydia, gonorrhoea, syphilis and trichomoniasis.
Where to look: Table 5, “Comparison of 2012 and 2016 WHO regional prevalence estimates of chlamydia, gonorrhoea, trichomoniasis and syphilis” — 2016 columns, women then men, for each WHO region
Used for: WHO-region prevalence of chlamydia, gonorrhoea, syphilis and trichomoniasis, which every country in this app inherits.
Where to look: “Key facts” and regional burden section — 47 million people with chronic hepatitis C in 2024; Eastern Mediterranean 12M, South-East Asia 9M, Europe 9M, Africa 8M, Western Pacific 7M, Americas 5M
Used for: WHO-region and global chronic hepatitis C prevalence.
https://www.who.int/news-room/fact-sheets/detail/hepatitis-c
Where to look: Indicator data table, filtered by country and by WHO region, latest available year
Used for: Adult (15–49) HIV prevalence per country and WHO region.
Where to look: Indicator data by WHO region and sex, 2020 — men/women: Africa 4.0/5.5, Americas 3.7/6.8, Eastern Mediterranean 3.5/4.4, Europe 2.0/3.4, South-East Asia 1.2/1.9, Western Pacific 2.3/4.3, global 2.5/4.0
Used for: Chlamydia prevalence by WHO region.
Where to look: Indicator data by WHO region and sex, 2020 — men/women: Africa 1.2/1.6, Americas 0.5/0.6, Eastern Mediterranean 0.4/0.5, Europe 0.2/0.3, South-East Asia 0.7/0.8, Western Pacific 0.7/0.9, global 0.7/0.8
Used for: Gonorrhoea prevalence by WHO region.
Where to look: Filter the returned rows on SpatialDim (WHO region code), Dim1 (SEX_MLE / SEX_FMLE) and TimeDim 2022. This indicator has no rendered page on the GHO portal, so the API endpoint is the citable location
Used for: Active syphilis prevalence by WHO region.
https://ghoapi.azureedge.net/api/STI_SYPHILIS_PREVALENCE_PER100
Where to look: Filter on SpatialDim (ISO3 country code or WHO region code), Dim1 = AGEGROUP_YEARSALL and TimeDim 2020; Low and High give the uncertainty interval. This indicator has no rendered page on the GHO portal, so the API endpoint is the citable location
Used for: Chronic hepatitis B prevalence for every country and WHO region in this app.
Where to look: Filter on SpatialDim (ISO3 country code) and TimeDim 2022. Values are published for 18 of this app's 21 countries; the United States, China and India are absent
Used for: Country-level chronic hepatitis C prevalence.
https://ghoapi.azureedge.net/api/HEPATITIS_HCV_PREVALENCE_PER100
Where to look: Abstract, Results — approximately 1.0% (95% CI 0.8–1.1) of US adults were HCV RNA-positive during 2013–2016, about 2.4 million people
Used for: United States hepatitis C prevalence, which WHO does not publish for that country.
Where to look: Table 1 — HSV-2 prevalence, ages 15–49, 2016, by WHO region and sex (women/men): Africa 43.9/25.4, Americas 24.0/11.6, Eastern Mediterranean 7.6/2.8, Europe 10.7/5.3, South-East Asia 9.6/7.2, Western Pacific 14.6/7.1, global 17.1/9.3 (13.2 overall)
Used for: HSV-2 prevalence by WHO region.
Where to look: Abstract, p. 1789 — global HPV prevalence 11.7% (95% CI 11.6–11.7); sub-Saharan Africa 24.0%, Eastern Europe 21.4%, Latin America 16.1%
Used for: Cervical HPV prevalence in women with normal cytology.
Where to look: Results, “Studies in the general population and community” — pooled prevalence 1.3% (95% CI 1.0–1.8) in higher-HDI countries and 3.9% (95% CI 2.2–6.7) in lower-HDI countries
Used for: Mycoplasma genitalium general-population prevalence.
Where to look: Total population by region, 2024 — approximately 1.20 billion (Africa), 1.04 billion (Americas), 2.11 billion (South-East Asia), 950 million (Europe), 790 million (Eastern Mediterranean), 1.99 billion (Western Pacific) and 8.16 billion worldwide, on the WHO regional groupings
Used for: The denominators used to turn WHO's regional hepatitis case counts into prevalence percentages.
Where to look: Table 1 — risk per 10 000 exposures: receptive anal 138 (95% CI 102–186), insertive anal 11 (4–28), receptive penile-vaginal 8 (6–11), insertive penile-vaginal 4 (1–14), receptive and insertive oral “low” (0–4)
Used for: Per-act HIV transmission probabilities for every activity.
Where to look: “Factors that modify per-act risk” — consistent condom use reduces HIV incidence by 80% (relative risk 0.20, 95% CI 0.08–0.47)
Used for: The condom reduction factor applied to the HIV transmission rates.
Where to look: Abstract — “the calculated risk of transmission per exposure with an infected partner was .19 for whites and .53 for blacks”
Used for: Per-act vaginal-to-penile gonorrhoea transmission.
Where to look: Abstract — infection followed 6 of 12 single exposures (50%) and 13 of 14 multiple exposures. The single-exposure figure rests on only 12 women
Used for: Per-act penile-to-vaginal gonorrhoea transmission.
Where to look: Model parameters — site-specific per-condomless-act transmission probabilities obtained by calibrating an individual-based model against MSM prevalence and behaviour data: 84% urethra-to-rectum, 2% rectum-to-urethra, 63% urethra-to-pharynx, 9% pharynx-to-urethra. These are fitted parameters, not direct observations
Used for: Per-act gonorrhoea transmission for anal and oral sex — the origin of the figures the Kirkcaldy review quotes.
Where to look: Discussion — site-specific transmission probabilities cannot be identified from population prevalence alone, and “additional data informing per act transmissibility are needed”
Used for: The caveat attached to the anal and oral gonorrhoea figures: they are model-fitted and weakly constrained.
Where to look: “Background”, p. 402 — penile-to-vaginal transmission ≈50% per act and vaginal-to-penile ≈20% per act; per-condomless-act oral (63% urethral-to-pharyngeal, 9% pharyngeal-to-urethral) and anal (84% urethral-to-rectal, 2% rectal-to-urethral)
Used for: The plain-language statement of the per-act gonorrhoea figures, and the pointer to the primary studies behind each.
Where to look: Section 5, “Data analysis and results” — per-encounter male-to-female transmission probability 0.129 (95% credible interval 0.096–0.206)
Used for: Per-act male-to-female chlamydia transmission.
Where to look: Results — per-partnership transmission: Natsal-2 male-to-female 32.1% (95% CrI 18.4–55.9) and female-to-male 21.4% (5.1–67.0); NHANES male-to-female 34.9% (22.6–54.9) and female-to-male 4.6% (1.0–13.1)
Used for: The direction ratio used to derive female-to-male chlamydia transmission.
Where to look: Abstract, “Results” — maximum incidence of HCV transmission by sex 0.07% per year (95% CI 0.01–0.13), approximately one transmission per 190 000 sexual contacts
Used for: Per-act hepatitis C transmission in heterosexual sex.
Where to look: Abstract and Results — 28.5 transmissions per 1000 unprotected acts from men to women and 1.7 per 1000 from women to men; 1.3 and 0.6 per 1000 protected acts respectively; condoms reduce per-act risk by 96% (men to women) and 65% (women to men)
Used for: Per-act HSV-2 transmission, with and without a condom.
Where to look: Abstract and Results — transmission rate 4.2 (95% PI 3.1–5.3) per 100 person-months overall; male-to-female 3.5 (2.5–4.7) and female-to-male 5.6 (3.8–7.9) per 100 person-months
Used for: HPV transmission rates between heterosexual partners.
Where to look: Abstract, Results — genital HPV incidence 37.8 per 100 patient-years at risk when partners used condoms for every act, against 89.3 when used for under 5% of acts (adjusted hazard ratio 0.3, 95% CI 0.1–0.6), i.e. a ~70% reduction
Used for: The condom reduction factor applied to the HPV transmission rates.
Where to look: Results, opening paragraph and Tables 1–2 — a median of 3 occasions of sex in the past month for both men and women in Natsal-3, with 14.4% of men and 13.2% of women reporting ten or more
Used for: The coital-frequency band the HPV per-act conversion is run across.
Where to look: “Syphilis” section — transmission requires direct contact with a syphilitic sore (chancre), and condoms prevent transmission only when the sore is covered
Used for: The route and condom caveat behind the syphilis transmission rates.
Where to look: “Trichomoniasis” section — T. vaginalis infects the vagina, urethra and paraurethral glands; oral and rectal infection is rare, so those sites are not tested or treated
Used for: Why the oral and anal trichomoniasis cells carry no per-act estimate.
Where to look: “Mycoplasma genitalium” section — M. genitalium causes urethritis and cervicitis and behaves epidemiologically like chlamydia; no per-act transmission probability has been established
Used for: The basis for the derived Mycoplasma genitalium transmission rates.
Where to look: “Chlamydial infections” and “Gonococcal infections” sections — both organisms infect mucosal surfaces and are transmitted during vaginal, anal or oral sex; no transmission route is described for contact that does not reach a mucosal surface
Used for: Why chlamydia and gonorrhoea are shown as negligible for genital rubbing and fingering.
Where to look: Abstract, Results — across 32 studies and 36 397 tested contacts, the pooled proportion of infected sexual contacts was 32.6% (95% CI 26.2–39.7), against the 60% that guidelines had assumed
Used for: The contact-level infection rate the syphilis per-act rates come from.
Where to look: Table 2, “Estimated per-act probability of acquiring HIV from an infected source, by exposure act” — receptive anal 138, insertive anal 11, receptive penile-vaginal 8, insertive penile-vaginal 4 per 10 000 exposures, the same values as Patel 2014
Used for: Independent confirmation that the HIV per-act figures used here are current CDC guidance.
Where to look: Transmission — “the most commonly reported route of transmission in Europe is through sexual transmission, when someone has sex without a condom with a person who has an HBV infection”. No per-exposure probability is given
Used for: Confirmation that sexual transmission of hepatitis B is a major route even though no per-act figure exists.
Where to look: “Transmission” section — HBV is transmitted by contact with blood or other body fluids, including semen and vaginal fluid, and is far more infectious than HIV per exposure
Used for: The relative infectivity used to derive hepatitis B per-act rates.
https://www.who.int/news-room/fact-sheets/detail/hepatitis-b
Where to look: Results and Figures 2–3 — sensitivity relative to patient infection status: chlamydia 65–100% (vaginal), 59–97% (cervical), 57–100% (urine); gonorrhoea 64–100% (vaginal), 85–100% (cervical), 67–94% (urine). Pooled estimates were not calculated
Used for: Chlamydia and gonorrhoea NAAT sensitivity.
Where to look: Tables 2–3 — TP-PA sensitivity 86.2–100% in primary syphilis with specificity 99.6–100%; treponemal immunoassays 94.5–100% sensitivity and specificity across stages
Used for: Syphilis serology sensitivity and specificity.
https://academic.oup.com/cid/article/71/Supplement_1/S13/5861811
Where to look: Table 4 — laboratory immunoassays for HBsAg: sensitivity 88.9% (95% CI 87.0–90.6) and specificity 98.4% (97.8–98.8) against a CMIA reference; rapid tests 90.0% (89.1–90.8) and 99.5% (99.4–99.5)
Used for: Hepatitis B (HBsAg) test sensitivity and specificity.
Where to look: Results, “Diagnostic accuracy” — pooled sensitivity 97% (95% CI 97–98) and specificity 99% (98–99) across all 52 studies
Used for: Hepatitis C antibody test sensitivity and specificity.
Where to look: “Genital herpes — type-specific serologic tests” — sensitivity of glycoprotein G type-specific tests for HSV-2 antibody varies from 80% to 98%, specificity is ≥96%, and false-positive results occur at low index values
Used for: HSV-2 serology sensitivity and specificity.
Where to look: “HIV infection — detection of HIV infection” — the recommended laboratory algorithm starts with an antigen/antibody combination immunoassay, which detects established infection with sensitivity and specificity >99%
Used for: HIV test sensitivity and specificity.
Where to look: Opening statistics — in 2022 the relative risk of acquiring HIV was 23 times higher for gay men and other men who have sex with men than for the wider adult male population
Used for: The HIV multiplier applied to male–male encounters.
https://www.unaids.org/sites/default/files/media_asset/2024-unaids-global-aids-update-gay-men_en.pdf
Where to look: Select gonorrhoea or syphilis, then stratify reported cases by transmission category — men who have sex with men account for the majority of male syphilis and gonorrhoea notifications in the EU/EEA
Used for: The share of syphilis and gonorrhoea notifications occurring in MSM, used to derive their multipliers.
Where to look: Abstract and Results — conception occurred only when intercourse took place during a six-day window ending on the day of ovulation, with day-specific probabilities rising from 0.10 five days before ovulation to 0.33 on the day itself
Used for: The pregnancy comparison risk.
Where to look: Abstract, Conclusions — the incidence of symptomatic influenza was approximately 8% per season, varying from 3% to 11% between seasons
Used for: The influenza comparison risk.
Where to look: “Scope of the problem” — 25.8% of women aged 15–49 who have been in a relationship have been subjected to physical and/or sexual violence by an intimate partner at least once; 31.6% of women worldwide (840 million) have experienced intimate partner or non-partner sexual violence in their lifetime
Used for: The intimate-partner-violence comparison risk.
https://www.who.int/news-room/fact-sheets/detail/violence-against-women
Used for: The “with condom” rates that are calculated by scaling the no-condom rate. HSV-2 is not among them — its protected rate was measured directly.
How this number was produced. Where a condom column is calculated rather than measured, it is the no-condom rate multiplied by (1 − reduction), using the published reduction for that infection: HIV 80% (Patel 2014, relative risk 0.20) and HPV 70% (Winer 2006, hazard ratio 0.3). HSV-2 is not calculated at all — Magaret 2016 measured the protected rate directly, so those cells are quoted. For the fluid-borne bacterial infections (chlamydia, gonorrhoea, Mycoplasma genitalium, trichomoniasis) and for hepatitis C, no per-act reduction has been published, so the HIV figure of 80% stands in as the closest measured analogue for a barrier that fully covers the exposed mucosa. Syphilis is the exception: transmission is by contact with a chancre a condom may not cover, so only a 50% reduction is applied, following the CDC statement that condoms protect only where they cover the sore. A condom is a barrier only for vaginal sex, anal sex and fellatio; for cunnilingus, genital rubbing and fingering the two columns are identical, because nothing is covered. Hepatitis B has no condom column because it has no per-act rate to reduce.
Used for: Chlamydia per-act rates for every direction other than penile-to-vaginal.
How this number was produced. Only the male-to-female per-act probability has been estimated directly: 12.9% (95% CrI 9.6–20.6) in Tu 2011. Lewis 2020 estimates per-partnership probabilities in both directions from two population surveys — female-to-male divided by male-to-female is 21.4/32.1 = 0.67 (Natsal-2) and 4.6/34.9 = 0.13 (NHANES). Applying that 0.13–0.67 ratio to the male-to-female per-act interval gives the female-to-male range: 9.6 × 0.13 = 1.3% to 20.6 × 0.67 = 13.8%. Receptive anal exposure is taken as equal to the receptive (male-to-female) figure and insertive anal as equal to the female-to-male figure, because in both cases the susceptible surface is respectively a columnar-epithelium mucosa and the male urethra; no direction-specific anal estimate exists.
Used for: Gonorrhoea per-act rates for anal and oral sex.
How this number was produced. The four figures — 84% urethra-to-rectum, 2% rectum-to-urethra, 63% urethra-to-pharynx, 9% pharynx-to-urethra — are used exactly as Hui 2015 reports them and as the Kirkcaldy review repeats them; no arithmetic is applied. They are listed as derived because of what they are rather than what was done to them: they are parameters fitted by calibrating a transmission model against prevalence and behaviour data in Australian men who have sex with men, not probabilities observed in couples. Spicknall 2019 re-examined the same model class and found such parameters cannot be pinned down from prevalence data alone, concluding that direct data on per-act transmissibility are still needed. Two consequences follow. The values carry far more uncertainty than their two-significant-figure appearance suggests, and they were fitted in a male population, so applying the urethra-to-pharynx figure to heterosexual fellatio assumes the anatomy governs transmission rather than the population. They are nonetheless the only per-act estimates that exist for these routes.
Used for: HIV per-act rates for oral sex.
How this number was produced. Patel 2014 reports receptive and insertive oral sex as “low”, with a confidence interval of 0–4 per 10 000 exposures rather than a point estimate. That interval is carried through directly as 0–0.04% per act. CDC’s 2025 post-exposure prophylaxis guideline reproduces the same table, so these remain current guidance rather than a single 2014 review. Cunnilingus is not separately estimated in that review and no case series supports a measurable per-act risk, so it is shown as negligible.
Used for: Hepatitis C per-act rates.
How this number was produced. Terrault 2013 reports approximately one transmission per 190 000 sexual contacts among monogamous heterosexual couples, which is 0.00053% per act; the confidence interval on the underlying annual incidence (0.01–0.13% per year against a point estimate of 0.07%) scales that to roughly 0.00008–0.00099% per act, used unchanged for vaginal sex in both directions and for insertive anal sex. No per-act figure exists for receptive anal sex, but hepatitis C outbreaks among men who have sex with men are concentrated there; the receptive-to-insertive ratio measured for HIV in Patel 2014 (138 versus 11 per 10 000, i.e. 12.5×) is applied to give 0.001–0.0124%. Oral and non-penetrative contact has no documented transmission route and is shown as negligible, as is every condom column, since 80% of an already sub-0.01% rate is far below the threshold at which a number would be meaningful.
Used for: Every syphilis per-act rate.
How this number was produced. No study reports a per-act probability for syphilis. What is measured is the proportion of a case's traced sexual contacts who turn out to be infected, pooled by Denman 2022 at 32.6% (95% CI 26.2–39.7). Two things make that a rough starting point rather than a transmission probability. It counts contacts found infected regardless of direction, so it includes people who infected the index case and people infected by someone else entirely; and contact tracing samples the partners of diagnosed cases, who are not a random sample of exposures. Converting it also depends on how many times a “contact” actually had sex, which those studies do not record. The range spans that last uncertainty at both ends: the low bound assumes a partnership of about ten acts at the lower confidence limit, 1 − (1 − 0.262)^(1/10) = 3.0%, and the high bound assumes the contact was a single act at the upper limit, 39.7%. It is a plausibility span, not a confidence interval. The same range applies to every route that brings skin or mucosa into contact with a lesion — vaginal, anal, oral and genital rubbing — because the mechanism gives no basis for ranking them, which almost certainly overstates the briefer contacts. Fingering is negligible because it brings no susceptible surface into contact with the sore. Beware of secondary sources quoting “51–64% per sexual encounter”: that is a per-partnership figure repeated as if it were per act.
Used for: HPV per-act rates.
How this number was produced. HPV transmission is measured per unit of time, not per act, so a per-act figure only exists once you assume how often a couple has sex — and that assumption, not the measurement, dominates the result. Malagón 2021 reports 3.5 (95% PI 2.5–4.7) transmissions per 100 person-months male-to-female and 5.6 (3.8–7.9) female-to-male. Natsal-3 found a median of 3 occasions of sex in the past month among adults in Britain, with 13–14% reporting ten or more, so the conversion is run across 3 to 12 acts per month rather than at a single figure. A monthly probability p over n acts gives 1 − (1 − p)^(1/n) per act: the male-to-female interval yields 0.21% at twelve acts and 1.59% at three, and the female-to-male interval 0.32% to 2.71%. Those spans are what the app shows, so they carry the frequency uncertainty rather than hiding it. Receptive anal exposure is taken as equal to the receptive genital figure and insertive anal as equal to the insertive figure. Oral sex and genital rubbing do transmit HPV, but no rate has been published for either and there is no measured ratio to scale the genital figure by, so those cells carry no estimate rather than an invented fraction. Fingering is negligible: a hand is not a susceptible surface.
Used for: Mycoplasma genitalium per-act rates.
How this number was produced. No per-act transmission probability has been established for M. genitalium. The CDC guidelines describe it as causing the same urethritis and cervicitis syndromes as chlamydia and spreading by the same route, so the chlamydia per-act rates are carried across unchanged for every activity — including the cells where chlamydia itself has no estimate (oral sex, cunnilingus) or is negligible (genital rubbing, fingering). This is an assumption of equivalence between two organisms, not a measurement of M. genitalium.
Used for: HSV-2 per-act rates for anal, oral and non-penetrative contact.
How this number was produced. Magaret 2016 measured only vaginal sex, in both the protected and unprotected columns: 2.85% per unprotected act to a susceptible woman and 0.17% to a susceptible man, falling to 0.13% and 0.06% with a condom. Anal sex reuses those figures unchanged, with the receptive partner taking the female value and the insertive partner the male value, because the exposure is the same shedding genital skin against a susceptible mucosa. Oral sex and genital rubbing are documented routes for HSV-2, but no rate has been published for either and there is no measured ratio to scale the genital figure by, so those cells carry no estimate. Fingering is negligible: a hand is not a susceptible surface, and no route is described for it. HSV-2 is shed intermittently, so a single-act figure understates the risk over a relationship and overstates it on a day with no shedding.
Calculated from source (25).
Used for: Trichomoniasis per-act rates.
How this number was produced. No per-act estimate exists for T. vaginalis, so the chlamydia per-act probabilities are used for vaginal sex in both directions, on the basis that both organisms infect the same urogenital surfaces. That substitution should be read as a floor rather than a best guess, and the reason is worth stating because it cuts against the assumption. Partner studies find T. vaginalis in roughly 30–70% of the male partners of infected women, which is at or above the 32% per-partnership figure measured for chlamydia — so trichomoniasis is, if anything, the more efficiently transmitted of the two. Those partner figures cannot be converted into a per-act rate (they are cumulative over a relationship, and the partners of diagnosed cases are not a random sample of exposures), which is why they are not used directly, but they mean the numbers here are more likely to understate than overstate. The CDC guidelines state that oral and rectal infection is rare enough that those sites are neither tested nor treated, so anal contact carries no estimate at all and oral contact, genital rubbing and fingering are shown as negligible.
Used for: Every multiplier applied when both participants are male.
How this number was produced. Prevalence figures are general-population averages, so a separate factor is applied when both participants are male. Only one of these factors comes from a published number: HIV uses the UNAIDS relative risk of 23. The rest are judgements, and it is worth being precise about how weak they are. ECDC surveillance shows men who have sex with men accounting for the majority of male syphilis and gonorrhoea notifications while making up an estimated 3–5% of the adult male population. Dividing a case share by a population share gives a concentration ratio of roughly 10 to 23 depending on which end of each estimate you take — and that is not the quantity wanted here. The rate ratio, which is what actually multiplies a prevalence, is higher again: 19 to 48 across the same inputs, because the non-MSM denominator shrinks as the MSM share grows. The 20 used for syphilis and 12 for gonorrhoea sit at or below the bottom of even the concentration range, chosen as deliberate floors rather than best estimates; syphilis is set higher than gonorrhoea because gonorrhoea retains a substantial heterosexual burden in the same surveillance that syphilis largely does not. Chlamydia, hepatitis B and HPV are set to 4, and hepatitis C, HSV-2 and M. genitalium to 3, reflecting the consistent direction of surveillance findings rather than any published ratio. Trichomoniasis is set to 1 because it is a urogenital organism that is rare in men who have sex with men. Two further caveats: a relative risk of acquiring an infection is not the same quantity as a ratio of prevalences, and the resulting prevalence is capped at 100%.
Used for: Any value where a country falls back to its WHO region, and every figure for chlamydia, gonorrhoea, syphilis, HSV-2, HPV, M. genitalium and trichomoniasis, none of which WHO estimates per country.
How this number was produced. WHO publishes country-level prevalence for HIV, hepatitis B and hepatitis C, and nothing per country for the other seven infections — those exist only at WHO-region resolution, so every country takes its region's figure for them. Two genuine data gaps sit inside the three that are published: the HIV indicator returns “no data” for every year for Poland, Ukraine, Türkiye, Israel, the United States, Japan and China, and the hepatitis C indicator has no row for China or India. The World Bank's mirror of the same UNAIDS series returns the identical fourteen countries, so this is a gap in what UNAIDS publishes rather than a gap in this lookup. Rather than substitute a figure from a source a reader cannot open, those cells inherit the WHO regional value and say so. That substitution can be materially wrong in both directions — Ukraine's national HIV figure is widely reported above the European regional average and Japan's far below the Western Pacific one — so anyone who needs a national number should take it from that country's own surveillance report. The sources listed below are every source a fallback value can come from, so whichever infection sent you here, the figure you are reading is one of them.
Calculated from sources (4), (9), (5), (6), (7), (8), (11), (12), (13), (2), (1), (60).
Used for: The hepatitis C percentages computed from regional case counts, plus the HPV and M. genitalium regions with no published estimate.
How this number was produced. Three regional values are computed here rather than quoted. Hepatitis C is published by WHO as a regional case count rather than a percentage, so each figure is that count divided by the region's population: 12 million chronic infections against about 790 million people in the Eastern Mediterranean Region gives 1.5%. HPV has a published figure only for sub-Saharan Africa (24.0%), eastern Europe (21.4%), Latin America (16.1%) and the world as a whole (11.7%); the Americas and Europe are given as the span between the global average and their regional anchor, and the Eastern Mediterranean, South-East Asia and Western Pacific regions take the global average, which each value says on its face. M. genitalium uses the higher-HDI pooled interval for Europe, the lower-HDI interval for Africa, the Eastern Mediterranean and South-East Asia, and both combined for the Americas and Western Pacific, which contain countries in each group. Everything else — chlamydia, gonorrhoea, syphilis, HIV, hepatitis B, HSV-2 and trichomoniasis — is quoted directly from a source.
Used for: The sensitivity and specificity applied when a recent negative test is entered.
How this number was produced. The reviews above mostly report ranges rather than single pooled values, so one representative figure per infection is chosen for a modern laboratory assay after the relevant window period. Chlamydia and gonorrhoea are the loosest: Rönn 2019 declined to pool, reporting sensitivity across studies of 57–100% and 64–100%, ranges so wide that their arithmetic midpoints (78% and 82%) would describe no assay actually in use. The 90% and 92% used here sit in the upper part of those ranges, where current commercial NAATs cluster, with specificity set at 99% since NAAT specificity is consistently at or above that. That is a judgement about which studies represent present-day practice, not a calculation. Syphilis: 93% sensitivity and 99.8% specificity, from the middle of the 86.2–100% and 99.6–100% TP-PA ranges in Park 2020. HIV: 99.5% and 99.9%, consistent with the CDC statement that the combination immunoassay exceeds 99% on both. HSV-2: 90% sensitivity from the CDC's stated 80–98% range and 96% specificity from its stated floor. Trichomoniasis and M. genitalium have no dedicated review here and take the chlamydia NAAT figures. HPV is a special case: it is not part of any standard panel, so a nominal 90%/90% is used, which deliberately leaves the estimate almost unchanged — a negative HPV result should not reassure, because the test was probably never done. Hepatitis B and C are quoted directly from Amini 2017 and Tang 2017 rather than derived. Every value assumes the infection was actually on the panel and that the test came after the window period, neither of which the app can verify.
Used for: The two pregnancy comparison risks shown next to the results.
How this number was produced. Wilcox 1995 measured the probability of conception by cycle day, not per act on an unknown day. Conception followed intercourse only within a six-day window ending at ovulation, with day-specific probabilities running from 0.10 five days out to 0.33 on the day itself. Averaging those six days across a 28-day cycle — the situation of someone not tracking ovulation — gives roughly 4% per act, which is the figure shown. It is an average over the cycle, so it is far too low for a mid-cycle act and far too high for most other days, which is what the note on the value says. The protected figure applies typical-use condom effectiveness of about 87% to that average, giving roughly 0.5%. These are comparisons for scale, not family-planning advice.
Calculated from source (45).
Headlines about STIs rarely tell you what a single encounter actually means. The STI Risks Calculator combines published per-act transmission rates with real prevalence data so you can compare the relative risk of chlamydia, gonorrhea, syphilis, HIV, hepatitis B and C, HPV, herpes, Mycoplasma genitalium and trichomoniasis side by side, for vaginal, anal, and oral sex, with or without a condom.
How likely a partner is to carry an STI depends heavily on where they live. The calculator covers 21 countries, including the Netherlands, France, Germany, the UK, Spain, Italy, the United States, Brazil, Australia, Japan and India, plus WHO regional averages, drawing on ECDC, UNAIDS and WHO surveillance data, so you can see how the same activity carries different risk in different places.
It's a free, private educational tool that compares published STI prevalence and per-encounter transmission rates so you can see how risk changes by country, partner, sexual activity, and condom use. It shows illustrative population averages, not a personal diagnosis or prediction.
The five most common (the 'Big 5': chlamydia, gonorrhea, syphilis, HIV, and hepatitis B/C), plus four supplementary infections: HPV, genital herpes (HSV-2), Mycoplasma genitalium, and trichomoniasis. Mpox is included as an awareness 'watch list' entry without per-encounter statistics, because it spreads in outbreaks rather than at a steady rate.
For each activity the illustrative risk is the published per-act transmission rate multiplied by the prevalence of that STI in the partner's country of residence, shown as a low-to-high range. Male-to-male contact applies an elevated-prevalence (MSM) adjustment, and you can replace country prevalence with a personalised estimate based on a recent negative test and partner history. Every number is traceable to a cited source (WHO, ECDC, UNAIDS, CDC, and peer-reviewed meta-analyses).
Yes. For fluid-borne infections such as HIV, gonorrhea, chlamydia and hepatitis B, condoms reduce per-encounter risk substantially. For skin-to-skin infections such as HPV and herpes they help less, because the virus can live on skin a condom doesn't cover. Toggle 'condom' in the calculator to compare both.
No. The STI Risks Calculator is an educational reference only. A low percentage is never a guarantee of safety. For testing, diagnosis, vaccination (for example hepatitis B or HPV), or treatment, speak to a healthcare professional.
No. There is no account, no database, and no analytics on your selections. Everything you enter, such as gender, country, activities, and test history, stays in your browser and is never sent to any server.