Where does the transmission probability data come from?

All per-act probability figures are drawn from CDC published estimates (CDC.gov HIV Transmission page). Numbers represent population-level averages and vary based on viral load, STI co-infection, circumcision status, and other factors.

Does "per-act risk" mean I will get HIV after one exposure?

No. Per-act risk is a statistical probability. A 1-in-900 risk (receptive anal intercourse without a condom) means that, on average, 1 in 900 such exposures results in transmission. Individual risk can be higher or lower depending on biological factors.

What is PrEP and does it appear in this tool?

PrEP (pre-exposure prophylaxis) is a daily antiretroviral medication that reduces HIV acquisition risk by up to 99% for sexual exposure when taken consistently. It is listed as a prevention option for relevant routes. Ask a clinician whether PrEP is right for you.

What should I do if I think I had a recent high-risk exposure?

Seek medical care as soon as possible — ideally within 72 hours. A clinician can prescribe PEP (post-exposure prophylaxis), a 28-day antiretroviral course that significantly reduces transmission risk when started early.

Is this tool a substitute for HIV testing?

No. Only an HIV test can confirm infection. If you have had a potential exposure, get tested at the appropriate window period (18–45 days for 4th-generation tests; 90 days for antibody-only tests). This tool is educational only.

Why do some routes show "negligible" risk?

Routes such as kissing, sharing food, or casual contact carry no documented transmission risk. CDC classifies these as negligible or zero based on decades of epidemiological evidence.

HIV Risk Assessment

Name: HIV Risk Assessment
Availability: InStock
Author: Nham Vu

Select an exposure route to see CDC-estimated per-act HIV transmission probability and risk-reduction options.

Educational reference only. Data from CDC per-act transmission estimates. Figures are population averages — individual risk depends on viral load, STI status, and other factors. Not a substitute for HIV testing or clinical advice.

Select an exposure route

Exposure route

Per-act probability

Risk tier

Relative risk (vs. highest)

Clinical context

Risk-reduction options

Recent high-risk exposure? PEP (post-exposure prophylaxis) must be started within 72 hours. Go to an emergency room or urgent care clinic now.

All routes at a glance

Exposure route	Per-act probability	Tier
Receptive anal intercourse (no condom)	1 in 72	highest
Insertive anal intercourse (no condom)	1 in 909	high
Receptive vaginal intercourse (no condom)	1 in 1,250	moderate
Insertive vaginal intercourse (no condom)	1 in 2,500	moderate
Receptive oral sex (fellatio)	1 in 10,000	low
Sharing injection needles (IDU)	1 in 160	highest
Needlestick injury (healthcare)	1 in 435	high
Blood transfusion (untested blood)	1 in 1.5	highest
Mother-to-child (without intervention)	~1 in 4	highest
Kissing / saliva contact	Negligible	negligible
Casual / household contact	Zero	negligible

Source: CDC.gov — HIV Transmission per-act risk estimates. "Negligible" and "Zero" routes have no documented transmission cases.

Summary

Select an exposure route to see CDC-estimated per-act HIV transmission probability and risk-reduction options.

How it works

Select the exposure route that applies to your situation.
Review the CDC-estimated per-act transmission probability.
Read the risk-reduction options shown for that route.
Use the comparison table to see risk across all routes at once.
Consult a healthcare provider for personal medical advice.

Use cases

Understand relative transmission risk before speaking with a clinician.
Compare risk levels across different exposure types.
Learn which prevention methods apply to each route.
Support sexual-health education and counseling sessions.
Reference CDC probability data without navigating lengthy reports.
Inform decisions about PEP (post-exposure prophylaxis) urgency.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu