VIRAL INFECTION / CARE & TRANSMISSION
Human immunodeficiency virus
HIV is preventable and very treatable. Someone on treatment who keeps an undetectable viral load does not pass HIV on through sex: U=U. For people without HIV, PrEP and condoms lower the chance of acquiring it, and PEP is an option soon after a possible exposure. [28][29][30]
Transmission risk
Risk by type of contact
Bottom line. HIV has the strongest route-specific transmission estimates in this matrix, but they apply only without effective prevention. Sustained viral suppression prevents sexual transmission, and PrEP and condoms materially change the baseline risk.
General
Oral
Vaginal
Anal
Kissing
What changes the risk (9)
- Effective viral suppression: U=U. HIGH: sustained viral suppression prevents sexual transmission. PARTNER2 observed no linked transmissions during 76,088 condomless anal acts; the upper 95% rate bound was 0.23 per 100 couple-years, not a per-act residual risk. Baseline unsuppressed estimates do not apply. Sources 5, 2
- Oral PrEP. HIGH when taken as prescribed: CDC estimates about 99% reduction in sexual acquisition. Adherence, drug/regimen and route eligibility matter; this is not a universal per-act efficacy estimate. Sources 2
- Lenacapavir PrEP: guidance changed in 2025. HIGH for the trials: PURPOSE 1 primary analysis IRR 0 (95% CI 0-0.04) and PURPOSE 2 IRR 0.04 (0.01-0.18) versus estimated background incidence, described as 100% and 96% efficacy. Breakthrough/postprimary infections occurred; do not promise 100% lifelong protection. CDC added this option in 2025. Sources 6
- PEP: updated 2025 guidance. MEDIUM for prevention evidence, LOW for an exact percentage: start as soon as possible and no later than 72 hours after a qualifying exposure; 28-day course. A defensible sexual-exposure efficacy percentage is unavailable. Sources 7
- Circumcision. HIGH for heterosexual male acquisition: randomized-trial evidence supports approximately halving HIV acquisition in circumcised men; not protection for receptive anal/vaginal partners or a replacement for PrEP/condoms. Sources 2
- Concurrent ulcerative STI. MEDIUM: RR 2.65 (95% CI 1.35-5.19) for acquisition with genital ulcer disease in a susceptible heterosexual partner. The estimate is not specific to every STI or directly measured in MSM. It does not negate U=U. Sources 1, 2
- Ejaculation during receptive anal intercourse. MEDIUM: Jin 2010 estimated 1.43% per act with ejaculation (95% CI 0.48-2.85) versus 0.65% with withdrawal (0.15-1.53). This was a fitted model from an Australian prospective cohort, not randomized withdrawal or uniformly measured source viral suppression; do not apply these figures to a suppressed source or as a universal twofold multiplier. Sources 8
- Menstruation. MEDIUM for the association, LOW for causal generalization: a 1992 European heterosexual-couple study found OR 3.4 (95% CI 1.0-11.1) for female-to-male infection associated with sex during menses. This partner-level, pre-modern-treatment association is not a per-act multiplier, nor evidence of risk under U=U. Sources 9
- Untreated viral load and infection stage. MEDIUM: a review reports RR 2.89 (95% CI 2.19-3.82) per log10 rise in source viral load, acute versus asymptomatic-stage ratio 7.25 (3.05-17.3), and late versus asymptomatic 5.81 (3.00-11.4). Stage estimates include derived historical hazard comparisons; correlated cofactors must not be multiplied as independent effects. None overrides sustained-suppression evidence. Sources 1
Condoms and dental dams
Heterosexual consistent-use reduction about 80% from cohort review; receptive anal 72% per person-year versus 91% per partnership from different analyses; insertive anal 63% per person-year. These are self-reported use, not perfect-use trials; no oral dental-dam efficacy percentage. Evidence: High Sources 2
Sources for this risk profile (9)
- Patel P et al. AIDS, 2014. Estimating per-act HIV transmission risk: a systematic review. DOI: 10.1097/QAD.0000000000000298. pmc.ncbi.nlm.nih.gov
- CDC CDC, 2024. HIV Risk and Prevention Estimates. cdc.gov
- CDC CDC, 2024. About STI Risk and Oral Sex. cdc.gov
- CDC CDC HIV Risk Reduction Tool, current web guidance (accessed 2026-10-27). Can I Get or Transmit HIV From...?. hivrisk.cdc.gov
- Rodger AJ et al. Lancet, 2019. Risk of HIV transmission through condomless sex in serodifferent gay couples with the HIV-positive partner taking suppressive antiretroviral therapy (PARTNER): final results. DOI: 10.1016/S0140-6736(19)30418-0. pubmed.ncbi.nlm.nih.gov
- Patel RR et al.; CDC MMWR, 2025. Clinical Recommendation for the Use of Injectable Lenacapavir as HIV Preexposure Prophylaxis - United States, 2025. DOI: 10.15585/mmwr.mm7435a1. cdc.gov
- CDC MMWR Recommendations and Reports, 2025. Antiretroviral Postexposure Prophylaxis After Sexual, Injection Drug Use, or Other Nonoccupational Exposure to HIV - United States, 2025. DOI: 10.15585/mmwr.rr7401a1. cdc.gov
- Jin F, Jansson J, Law M et al. AIDS, 2010. Per-contact probability of HIV transmission in homosexual men in Sydney in the era of HAART. DOI: 10.1097/QAD.0b013e3283372d90. pmc.ncbi.nlm.nih.gov
- European Study Group on Heterosexual Transmission of HIV BMJ, 1992. Comparison of female to male and male to female transmission of HIV in 563 stable couples. DOI: 10.1136/bmj.304.6830.809. 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
Testing time depends on the assay: lab fourth-generation 18-45 days; rapid and antibody-only tests can need up to 90 days.
When to be seen sooner. After a possible HIV exposure, PEP works best started as soon as possible and within 72 hours, so contact a clinic or emergency service promptly rather than waiting for a test window.
-
Laboratory fourth-generation HIV antigen/antibody test
CDC usual detection range: 18-45 days after exposure, in the absence of antiretroviral medications.
The lower end is not a reliable rule-out day. Follow the assay-specific repeat plan after an early negative.
Specimens: Venous blood
Sources 1, 2, 3
-
Rapid antigen/antibody test
CDC usual detection range: 18-90 days after exposure, in the absence of antiretroviral medications.
The lower end is not a reliable rule-out day. Follow the assay-specific repeat plan after an early negative.
Specimens: Finger-stick blood
Sources 1, 2, 3
-
Antibody-only test, including many self-tests
CDC usual detection range: 23-90 days after exposure, in the absence of antiretroviral medications.
The lower end is not a reliable rule-out day. Follow the assay-specific repeat plan after an early negative.
Specimens: Blood or oral fluid, depending on assay
Sources 1, 2, 3
-
HIV nucleic acid test (NAT)
CDC usual detection range: 10-33 days after exposure, in the absence of antiretroviral medications.
The lower end is not a reliable rule-out day. Follow the assay-specific repeat plan after an early negative.
Specimens: Blood
Sources 1, 2, 3
Early negatives, after treatment, and screening limits
An early negative result. Repeat after the relevant test window when an earlier test is negative and consider all subsequent exposures. With PEP/PrEP, use clinician-directed antigen/antibody plus NAT testing; do not apply the unmedicated ranges as an automatic clearance date.
After treatment. After nPEP, CDC recommends interim antigen/antibody plus NAT at 4-6 weeks and final testing at 12 weeks. Its summary anchors to exposure; its detailed follow-up text anchors to PEP initiation. Keep the treating service's date and do not silently substitute treatment-completion timing. No test-of-cure applies to established HIV.
Screening limits. PEP and PrEP, including long-acting drugs, can delay detectability. A baseline test shortly after exposure cannot rule out that exposure. NAT is not a substitute for following the full diagnostic algorithm.
Kissing and body sites. The assay window is not different because the contact was oral rather than vaginal/anal. Whether that contact warrants testing or PEP is a separate exposure assessment; ordinary kissing alone is not a routine HIV-exposure indication.
Sources for testing guidance (5)
- CDC. HIV Testing: window periods by assay. 2025. cdc.gov
- CDC. Getting Tested for HIV. 2025. cdc.gov
- Delaney KP et al., Clinical Infectious Diseases. Time Until Emergence of HIV Test Reactivity Following Infection With HIV-1. 2017. DOI: 10.1093/cid/ciw666. pubmed.ncbi.nlm.nih.gov
- CDC, MMWR. Antiretroviral Postexposure Prophylaxis After Nonoccupational Exposure to HIV, United States, 2025. 2025. DOI: 10.15585/mmwr.rr7401a1. cdc.gov
- CDC. How HIV Spreads. 2024. cdc.gov
Treatment
Antiretroviral therapy suppresses the virus, protects the immune system and prevents sexual transmission when suppression is sustained. It is ongoing treatment, not a routine cure. PrEP is prevention for people without HIV; available options include oral and long-acting regimens, including six-monthly lenacapavir in the US. [28][29][31]
Actions
1. Within 72 hours, ask about PEP. After a possible exposure, contact a clinic or emergency service as soon as you can. PEP works best started early, within 72 hours, and is usually taken for 28 days. There is no need to wait for a partner's test result or for symptoms. [30]
2. Match testing to timing. A laboratory antigen/antibody test usually detects infection 18-45 days after exposure; other tests have different windows. PrEP or PEP can change follow-up. A clinician may add an HIV RNA test. [30][33]
3. Plan prevention and partners. PrEP is worth discussing if exposures are ongoing, along with regular STI testing by site. A new diagnosis is followed by care and, if wanted, confidential partner services; how far back to look depends on when infection was likely, not one fixed rule. [29][30][61]
SCIENCE & CONTEXT
HIV / the evidence
History
AIDS was first formally recognized in 1981; HIV was identified in the following years. Combination antiretroviral therapy transformed prognosis in the 1990s. Subsequent prevention studies established treatment as prevention, while oral and long-acting PrEP expanded options. Discovery dates do not imply that the virus first entered humans in 1981. [31][32]
Impact
HIV targets immune cells, including CD4 T cells. Without treatment it gradually weakens the immune system; with treatment, people live long, healthy lives. Early infection may feel like a passing viral illness or cause no symptoms, so testing, not appearance, is how it is diagnosed. [28][30][32]
HIV does not spread through ordinary social contact. The clinically relevant source state is not simply "HIV positive": sustained suppression removes sexual transmission risk. A population HIV-prevalence number that ignores suppression is therefore unsuitable as a direct transmission calculator input. [28]
Probability at a glance
| Sex act | No prevention | Consistent-condom model |
|---|---|---|
Receptive anal | 1.38% | 0.39% |
Insertive anal | 0.11% | 0.041% |
Receptive vaginal | 0.08% | 0.016% |
Insertive vaginal | 0.04% | 0.008% |
Sustained viral suppression | 0 sexual transmission | U=U |
Per-act percentages are conditional on a source partner with HIV without sustained suppression. Condom estimates apply CDC's observed consistent-use modifiers, not verified perfect use. U=U supersedes these sexual-risk estimates when suppression is sustained. [27][28]
PrEP reduces sexual acquisition risk by about 99% when used as prescribed. Do not multiply prevention percentages mechanically across studies with different populations or endpoints. [29]