The numbers your doctor won't tell you.

Evidence-based STI transmission probabilities. Interactive risk calculators. Complete transparency in methodology. Make informed decisions about your sexual health.

Calculate Your Risk Browse STI Data
8 STIs Covered
20 Studies Cited
∞ Scenarios
The Problem

Why I Built This

I'm frustrated. Every time I ask a doctor about STI transmission probabilities, I get the same non-answers: "Use a condom and you'll be fine" or "Don't worry about it."

That's not good enough. If I have a partner with an STI, I want to know the actual numbers. What's my probability of transmission per encounter? How does that change with condom use? What if we're together for a year versus a month?

Doctors seem to operate on a culture of managed information — deciding what you "need to know" rather than giving you the data to make your own informed decisions. With HPV, for example, the medical establishment has largely planned for it to spread through the population rather than helping individuals understand and manage their risk.

This site changes that. Every number here comes from peer-reviewed studies. Every calculation is transparent. You can see the equations, adjust the variables, and understand exactly how your risk is computed.

Disclaimer: This is not medical advice. This is mathematical modeling based on published research. Consult healthcare providers for personal medical decisions.

Our Principles

  • 📚
    Cite Everything

    Every transmission rate links to its source study

  • 🔢
    Show the Math

    All equations displayed in notation and code

  • 🎛️
    Your Variables

    Customize calculations to your situation

  • ⚠️
    Honest Uncertainty

    We show confidence intervals, not false precision

Interactive Tool

Risk Calculator

Select an STI, input your scenario, and see your cumulative risk over time.

1×/week Daily 3×/day
1 month 5 years
Per-Act Transmission Rate
Without condom: --
With condom: --
Cumulative risk after 12 months: --

Take Action

Testing Guide

Know when to test, what to ask for, and what a "standard panel" actually includes.

🕐 Window Periods

You can't test the day after exposure. Each STI has a window period — the time between infection and when a test can detect it. Testing too early = false negative.

STI Window Period Test Type
HIV 18–45 days (lab); 18–90 days (rapid)CDC HIV Testing"An antigen/antibody lab test using blood from a vein can usually detect HIV 18 to 45 days after exposure ... A rapid antigen/antibody test done with blood from a finger stick can usually detect HIV 18 to 90 days after exposure"View Source → Blood
Chlamydia 2 weeksNHS Sexual Health Oxfordshire"We usually say to wait ... 2 weeks for chlamydia and gonorrhoea"View Source → Urine or swab
Gonorrhea 2 weeksNHS Sexual Health Oxfordshire"We usually say to wait ... 2 weeks for chlamydia and gonorrhoea"View Source → Urine or swab
Syphilis 4 weeksNHS Sexual Health Oxfordshire"We usually say to wait ... 4 weeks for syphilis and HIV (in some circumstance you also need a test at 3 months)"View Source → Blood
Herpes (HSV) Up to 16 weeks (antibody test)CDC Herpes Testing"After exposure, it can take up to 16 weeks or more for current tests to detect infection"View Source → Blood or swab active sore
HPV No routine test existsCDC About HPV"There is no test to find out a person's 'HPV status.' ... There are HPV tests that can screen for cervical cancer. Healthcare providers only use these tests for screening women aged 30 years and older."View Source → Pap smear (women 30+ only)
Trichomoniasis No defined window periodASHM Contact Tracing Guidelines"Incubation period ... There is insufficient data to provide a definitive period"⚠️ NAAT testing can detect shortly after infection, but no established window period exists.View Source → NAAT (urine or swab)
Hepatitis B ~4 weeks average (range 1-9 weeks)CDC/Medical SourcesHBsAg can be detected as early as 1 week and as late as 9 weeks after exposure. Average detection occurs around 4 weeks.⚠️ Source not yet verified in test suite. Blood (HBsAg test)

⚠️ What's NOT in a Standard Panel

When you "get tested for everything," you probably weren't tested for everything. Standard STI panels vary by provider, but these are commonly excluded:

STI Why It's Usually Excluded How to Get Tested
Herpes (HSV-1 & HSV-2) CDC does not recommend routine testing for people without symptomsCDC Herpes Screening"CDC does not recommend herpes testing for people without symptoms in most situations. This is because of the limits of a herpes blood test and the possibility of a wrong test result."View Source → Ask specifically; blood test or swab active sore
HPV No general test exists to find "HPV status"CDC About HPV"There is no test to find out a person's 'HPV status.' ... There are HPV tests that can screen for cervical cancer. Healthcare providers only use these tests for screening women aged 30 years and older."View Source → Cervical Pap smear (women 30+); visual inspection for warts
HIV Some clinics require opt-in or separate consent Request specifically; many clinics offer free/confidential testing
Trichomoniasis Not nationally reportable; no routine screening recommendationsCDC STI Treatment Guidelines"Because trichomoniasis is not a reportable disease, and no recommendations are available for general screening for T. vaginalis, the epidemiology of trichomoniasis has largely come from population-based and clinic-based surveillance studies."View Source → Request NAAT test specifically (urine or swab)

💡 Bottom line: Always ask your provider "What exactly am I being tested for?" Don't assume "full panel" means everything.

The Data

STI Profiles

Click any STI for detailed transmission data, risk factors, and sources.

Each STI shows three key facts: how long it lasts, what happens without treatment, and what happens with treatment.

Duration

  • Curable — Gone after treatment
  • Usually Clears — Body fights it off
  • Chronic — Lifelong

If Untreated

  • What happens if you ignore it
  • Ranges from mild to life-threatening

With Treatment / Prevention

  • What happens when you take action
  • Usually much better than untreated

HIV

Duration Chronic
If Untreated Fatal (AIDS)
With Treatment Normal lifespan
  • 🔬 Low per-act transmission but serious consequences
  • 💊 Undetectable = Untransmittable (U=U)CDC HIV Risk and Prevention Estimates"no risk of sexual transmission. This translates to an effectiveness estimate of 100%"View Source →
  • 🛡️ Condoms reduce risk by 80%CDC HIV Risk and Prevention Estimates"Always using condoms, based on self-report, during sex with an HIV-positive partner reduces the risk of HIV acquisition by an estimated 80% among heterosexual men and women"View Source →
  • 💉 PrEP reduces risk by ~99% when taken dailyCDC HIV Risk and Prevention Estimates"the risk of acquiring HIV is reduced by about 99% among MSM"View Source →
  • ⚠️ Higher risk with other STIs present (inflammation)

Transmission Rates (per vaginal sex act)

Direction No Condom Condom Effectiveness
Male → Female 0.08%CDC HIV Risk and Prevention Estimates"Risk per 10,000 exposures ... Receptive penile-vaginal intercourse ... 8"View Source → 80% reductionCDC HIV Risk and Prevention Estimates"Always using condoms, based on self-report, during sex with an HIV-positive partner reduces the risk of HIV acquisition by an estimated 80% among heterosexual men and women"View Source →
Female → Male 0.04%CDC HIV Risk and Prevention Estimates"Risk per 10,000 exposures ... Insertive penile-vaginal intercourse ... 4"View Source →
Key Context

HIV transmission probability varies significantly based on viral load, presence of other STIs, and stage of infection. The rates above assume a partner with detectable (untreated) HIV. Partners on effective antiretroviral therapy with undetectable viral load have essentially zero transmission risk (U=U: Undetectable = Untransmittable).

PrEP: Pre-exposure prophylaxis reduces risk by ~99% when taken daily. PEP: Post-exposure prophylaxis can prevent infection if started within 72 hours.

Herpes (HSV-2)

Duration Chronic
If Untreated Periodic outbreaks
With Treatment Rare/no outbreaks
  • 🔄 Transmits via skin-to-skin contact, not just fluids
  • 😶 Most transmissions occur while asymptomaticCDC STD Treatment Guidelines"most genital herpes infections are transmitted by persons unaware that they have the infection or who are asymptomatic when transmission occurs"View Source →
  • 🛡️ Condoms reduce risk by 96% M→F, 65% F→MMagaret et al. 2016 (Clin Infect Dis)"condom use reduced per-act risk of transmission from men to women by 96% ... and marginally from women to men by 65%"View Source →
  • 💊 Daily antivirals (e.g., valacyclovir) reduce transmission by ~50%Corey et al. 2004 (NEJM)"acquisition of HSV-2 was observed in 14 of the susceptible partners who received valacyclovir (1.9 percent), as compared with 27 (3.6 percent) who received placebo"Reduction: (3.6% - 1.9%) / 3.6% ≈ 47-50%View Source →
  • 📅 Avoiding sex during outbreaks significantly reduces risk

Transmission Rates (per vaginal sex act)

Direction No Condom Condom Effectiveness
Male → Female 0.053%Corey et al. 2004 (NEJM) — Derived"The overall rate of transmission of HSV-2 was 3.6 per 100 person-years" — Derived to per-act assuming ~70 acts over 8-month study periodView Source → 96% reductionMartin et al. 2009 (PMC) — From Table 1Table 1: "Use of condoms" → Risk Ratio = 0.04 for susceptible women (1 - 0.04 = 96% reduction)View Source →⚠️ Data from table, not prose text
Female → Male 0.053%Corey et al. 2004 (NEJM) — Derived"The overall rate of transmission of HSV-2 was 3.6 per 100 person-years" — Derived to per-act (study didn't distinguish direction)View Source → 65% reductionMartin et al. 2009 (PMC) — From Table 1Table 1: "Use of condoms" → Risk Ratio = 0.35 for susceptible men (1 - 0.35 = 65% reduction)View Source →⚠️ Data from table; p=.060 (not statistically significant)

Note: Per-act rate derived from 8-month study. HSV-2 sheds asymptomatically, so transmission can occur without visible sores. Daily antivirals reduce transmission by ~50% on top of condoms.

Key Context

HSV-2 transmission is complex because the virus sheds asymptomatically. Even without visible sores, transmission can occur. Avoiding sex during outbreaks significantly reduces risk.

Daily antivirals (e.g., valacyclovir) reduce transmission by ~50% on top of condom use. Most people with HSV-2 are asymptomatic or have mild symptoms they don't recognize as herpes.

💡 What about HSV-1 (oral herpes)?

HSV-1 typically causes cold sores and affects ~67% of people globally — most acquired non-sexually in childhood. However, HSV-1 can be transmitted to the genitals through oral sex. Genital HSV-1 infections are increasingly common but tend to cause fewer outbreaks than genital HSV-2.

HSV-1 is not included in the risk calculator because: (1) most transmission is non-sexual, and (2) oral-to-genital transmission rates are not well-quantified in the literature.

HPV (Human Papillomavirus)

Duration Usually clears on its own
If Untreated Some strains cause cancer
Prevention Vaccine highly effective
  • 📊 Most sexually active people get HPV at some pointChesson et al. 2014"average lifetime probability of acquiring HPV among those with at least 1 opposite sex partner to be 84.6% ... for women and 91.3% ... for men"View Source →
  • 🧬 200+ strains; most are harmless, some cause cancer
  • 🛡️ Condoms reduce risk by 70% with consistent useMDedge"Women whose sexual partners consistently used condoms were 70% less likely to acquire genital human papillomavirus infection than were those whose partners did not"View Source →
  • 💉 Vaccine prevents the most dangerous strains
  • ⏳ 90% of infections clear within 2 years

Transmission Rates (per vaginal sex act)

Direction No Condom Condom Effectiveness
Male → Female 0.41%Malagón et al. 2021 (HITCH cohort) — Derived"The transmission rate from men to women was 3.5 ... per 100 person–months"View Source → 70% reductionMDedge / Winer et al."Women whose sexual partners consistently used condoms were 70% less likely to acquire genital human papillomavirus infection"View Source →
Female → Male 0.66%Malagón et al. 2021 (HITCH cohort) — Derived"... from women to men was 5.6 (95%PI 3.8–7.0) per 100 person–months ... rate ratio of 1.6 (95%PI 1.0–2.5)"View Source →

Per-act rates derived from person-month data. HPV is highly variable — most sexually active people get it at some point. Vaccine is the most effective prevention.

Key Context

HPV is unique because it's so common that "preventing" it may be unrealistic for sexually active adults who aren't vaccinated. The medical establishment's approach has largely been to expect exposure and rely on the immune system to clear it.

The key questions: (1) Are you vaccinated? (2) Are you getting regular screenings (Pap smears for cervix-havers)? The vaccine prevents the strains that cause most cancers and genital warts.

Chlamydia

Duration Curable
If Untreated PID, infertility risk
With Treatment Cured, no lasting effects
  • 🦠 Most common bacterial STI
  • 😶 ~70% of women and 50% of men have no symptomsFinnish Student Health Service"Chlamydia is often symptomless: about 50% of men and 70% of women have no symptoms"View Source →
  • 💊 Easily cured with antibiotics (usually azithromycin)
  • 🛡️ Condoms reduce risk by 60%Crosby et al. 2004 (JAMA Pediatrics)"We found that correct and consistent use of condoms reduced the risk of chlamydial infection by 60%"View Source →
  • ⚠️ Untreated can cause PID, infertility

Transmission Rates (per vaginal sex act)

Direction No Condom Condom Effectiveness
Male → Female 11.4%NCBI Book NBK261441 — Derived"the per-sex act transmission probability for chlamydia ... is in the range 6–16.7%" — Midpoint: 11.4%View Source → 60% reductionCrosby et al. 2004 (JAMA Pediatrics)"We found that correct and consistent use of condoms reduced the risk of chlamydial infection by 60%"View Source →
Female → Male 11.4%NCBI Book NBK261441 — Derived"the per-sex act transmission probability for chlamydia ... is in the range 6–16.7%" — Midpoint (no M→F vs F→M distinction in source)View Source →

Per-act rate is midpoint of 6-17% range. Source does not distinguish M→F vs F→M.

Key Context

Chlamydia is extremely common and often asymptomatic, which is why routine testing is recommended. It's easily treated but can cause serious complications if left untreated (pelvic inflammatory disease, infertility).

Testing: Recommended annually for sexually active women under 25 and anyone with new or multiple partners. Treatment is simple: a single dose of azithromycin or a week of doxycycline.

Gonorrhea

Duration Curable
If Untreated Spreads, infertility risk
With Treatment Cured, no lasting effects
  • 🦠 Second most common bacterial STI
  • ⚠️ Increasing antibiotic resistance is a global concernKirkcaldy et al. 2019"successively acquired antimicrobial resistance to each antimicrobial agent used and recommended for treatment ... The confluence of emerging resistance to cephalosporins and macrolides"View Source →
  • 💊 Current treatment: ceftriaxone injection
  • 🛡️ Condoms reduce risk by 90%Crosby et al. 2004 (JAMA Pediatrics)"We found that correct and consistent use of condoms reduced the risk of gonorrhea by 90%"View Source →
  • 🔥 More likely to cause symptoms than chlamydia (burning, discharge)

Transmission Rates (per vaginal sex act)

Direction No Condom Condom Effectiveness
Male → Female 22.8%NCBI Book NBK261441 — Derived"the per-sex act transmission probability for gonorrhoea is assumed to be twice that of chlamydia" (chlamydia 6-16.7%, midpoint 11.4% × 2 = 22.8%)View Source → 90% reductionCrosby et al. 2004 (JAMA Pediatrics)"We found that correct and consistent use of condoms reduced the risk of gonorrhea by 90%"View Source →
Female → Male 22.8%NCBI Book NBK261441 — Derived"the per-sex act transmission probability for gonorrhoea is assumed to be twice that of chlamydia" — Source does not distinguish M→F vs F→MView Source →

Per-act rate derived from 2× chlamydia. Source does not distinguish M→F vs F→M.

Key Context

Gonorrhea is highly infectious — even a single unprotected encounter carries significant risk. Unlike chlamydia, gonorrhea often causes noticeable symptoms (burning, discharge).

Antibiotic resistance is a growing concern. Current treatment requires a ceftriaxone injection. If you've had gonorrhea, get retested 3 months after treatment to ensure it's cleared and you haven't been reinfected.

Syphilis

Duration Curable
If Untreated Organ damage, death
With Treatment Cured, no lasting effects
  • 📈 Cases are surging after decades of decline
  • 🔴 Primary stage: painless sore (chancre)
  • 💊 Easily cured with penicillin
  • 🛡️ Condoms reduce risk by 50-71%PMC Condom Review"syphilis transmission is reduced 29% for typical use ... It is reduced 50-71% when condoms are used 100% of the time correctly"View Source → (sores may be outside covered area)
  • ⚠️ Untreated can cause serious organ damage years later

Transmission Rates (per vaginal sex act)

Stage No Condom Condom Effectiveness
Early syphilis (primary, secondary, early latent) >20%ASHM Contact Tracing Guidelines"The overall probability of transmission of syphilis during early infection is estimated to be greater than 20% per contact"View Source → 50-71% reductionPMC4660551 — Condom Review"Syphilis transmission is reduced 29% for typical use. It is reduced 50–71% when condoms are used 100% of the time correctly."View Source →
Latent stage Low⚠️ Needs verification — not well quantified in literature

Transmission depends heavily on stage. Primary/secondary stages are highly infectious. Condoms less effective than for other STIs because sores may be outside covered area.

Key Context

Syphilis progresses through stages, and transmission risk varies dramatically. The primary chancre (sore) is the most infectious period. If you can see a sore, avoid contact — that's when transmission is most likely.

Syphilis is resurging after decades of decline. Get tested if sexually active, especially with new partners. Treatment is simple: a penicillin injection cures it at any stage, though damage from late-stage syphilis cannot be reversed.

Hepatitis B

Duration Usually clears (~95% of adults recover)
If Untreated Liver cancer, cirrhosis, death
Prevention Vaccine is highly effective
  • 💉 Vaccine-preventable — most effective prevention method
  • ⚠️ More infectious sexually than HIV (per-partnership rates 25-44%)
  • 🛡️ Condoms provide >90% protectionPMC Condom Review"The condom offers maximum protection (more than 90%) against HIV, Hepatitis B virus and N.Gonorrhoea"View Source →
  • 📊 ~95% of infected adults recover completely (only ~5% become chronic)CDC Hepatitis B Clinical Overview"90% of infected infants ... will remain chronically infected with HBV. By contrast, approximately 95% of infected adults recover completely"View Source →
  • ⚠️ Infants: 90% become chronic — vaccination at birth is critical

Transmission Rates

Without Condom
Per partnership 25–44%Multiple SourcesPer-partnership transmission rates range from 18-44% depending on viral load. Per-act rates vary widely based on HBeAg status.⚠️ Per-act rate not reliably quantified. Risk is MUCH higher if partner has high viral load (HBeAg positive).
With Condom
Reduction >90%PMC Condom Review"The condom offers maximum protection (more than 90%) against HIV, Hepatitis B virus and N.Gonorrhoea"View Source →
View Full Details

Key Context

Hepatitis B is NOT Hepatitis C. People confuse these constantly. Hep B is sexually transmitted and quite infectious. Hep C is primarily blood-borne and almost never transmitted through heterosexual vaginal sex (~1 in 190,000 per contact).

Vaccination is key. The Hep B vaccine is extremely effective and is now routinely given to infants. If you weren't vaccinated as a child, you should get vaccinated — especially before new sexual partners.

Why no calculator? Per-act transmission rates vary enormously based on the partner with Hep B's viral load (HBeAg status). Someone with active viral replication is far more infectious. This makes a simple calculator misleading.

Trichomoniasis

Duration Curable
If Untreated Pregnancy complications, HIV risk
With Treatment Cured with single dose
  • 🥇 Most common curable STI (~2.6 million US cases)CDC STI Treatment Guidelines"Trichomoniasis is estimated to be the most prevalent nonviral STI worldwide, affecting approximately 2.6 million persons in the United States"View Source →
  • 😶 70-85% have no symptomsCDC STI Treatment Guidelines"The majority of persons who have trichomoniasis (70%–85%) either have minimal or no genital symptoms, and untreated infections might last from months to years"View Source →
  • 💊 Easily cured with one dose of metronidazole or tinidazole
  • 🛡️ Condom effectiveness controversial (~30% reduction observed)PMC Condom Review"It is claimed to provide more than 90% protection against Trichomonas vaginitis, but the latest reports are controversial. Statistically significant reduction in trichomonas infection (30%) was reported with condom use."⚠️ Earlier claims of >90% protection are disputed. More recent studies show only ~30% reduction.View Source →
  • 🌍 156 million new cases globally per yearWHO Fact Sheet"In 2020 there were approximately 156 million new cases of T. vaginalis infection among people aged 15–49 years old"View Source →
  • ⚠️ Increases HIV acquisition risk by ~1.5xCDC STI Treatment Guidelines"T. vaginalis infection is associated with a 1.5-fold increased risk for HIV acquisition and is associated with an increase in HIV vaginal shedding"View Source →

Transmission Rates

Without Condom
Vaginal sex UnknownASHM Contact Tracing Guidelines"Likelihood of transmission per act of unprotected intercourse ... Unknown, likely moderate to high"⚠️ No quantified per-act rate exists in medical literature. Cannot be used in risk calculator.View Source →
With Condom
Reduction 30–90% reduction (disputed)PMC Condom Review"It is claimed to provide more than 90% protection against Trichomonas vaginitis, but the latest reports are controversial. Statistically significant reduction in trichomonas infection (30%) was reported with condom use."⚠️ Condom effectiveness for trichomoniasis is controversial. Earlier claims of >90% are disputed by newer studies showing ~30%.View Source →
View Full Details

Key Context

Trichomoniasis is often called the "neglected STI" — it's the most common curable sexually transmitted infection, yet it receives far less attention than chlamydia or gonorrhea. Unlike those, it's not nationally reportable in the U.S.

Why can't we calculate risk? Despite being extremely common, there are no published per-act transmission rates for trichomoniasis. Medical literature describes it as "moderate to high" but provides no specific numbers. This is a gap in sexual health research.

Testing: Trichomoniasis is often NOT included in standard STI panels. You may need to specifically request testing. Diagnosis is via NAAT (urine or swab). Treatment is simple: a single oral dose of metronidazole or tinidazole cures ~95% of cases.

Transparency

Methodology

How we calculate transmission probabilities and where the numbers come from.

The Core Equation

All risk calculations use the same fundamental probability formula. If each exposure has independent risk \( r \), then after \( n \) exposures:

\[ P_{transmission} = 1 - (1 - r)^n \]

Intuitively: we calculate the probability of not getting infected after each encounter, multiply them together, then subtract from 1.

JavaScript Implementation

function calculateCumulativeRisk(perActRisk, numberOfEncounters) {
    // Probability of NOT being infected after all encounters
    const probNotInfected = Math.pow(1 - perActRisk, numberOfEncounters);
    
    // Probability of being infected = 1 - (prob of not infected)
    return 1 - probNotInfected;
}

// Example: Chlamydia (12.5% per act), 8 encounters
calculateCumulativeRisk(0.125, 8);  // Returns: 0.656 (65.6%)

Adjusting for Condom Use

Condoms reduce transmission by a certain percentage, depending on the STI. We multiply the base risk by (1 - effectiveness):

\[ r_{protected} = r_{unprotected} \times (1 - e_{condom}) \]

function adjustForCondom(baseRisk, condomEffectiveness) {
    return baseRisk * (1 - condomEffectiveness);
}

// HIV: 0.08% base, 80% condom effectiveness
adjustForCondom(0.0008, 0.80);  // Returns: 0.00016 (0.016%)

Limitations & Caveats

🎯 Individual Variation

Risk varies by viral load, inflammation, other infections, and biological factors. Population averages may not reflect your specific risk.

📊 Study Populations

Studies often focus on specific populations (e.g., discordant couples, clinic attendees). Results may not generalize to all scenarios.

🔄 Independence Assumption

The formula assumes each encounter is independent. In reality, factors like mucosal trauma or co-infections can create dependencies.

📅 Data Age

Some studies are older. HIV treatment has improved dramatically; resistance patterns change. We cite dates so you can assess currency.