Short answer
Relative STI risk forms a ladder with five rungs. Impossible, negligible, moderate, high, and the modifiers that move acts between them.
Casual contact (toilets, pools, hugging, food, mosquitoes, dried blood) sits at impossible: no STI route exists.
Correct consistent condoms and tested mutual monogamy sit at negligible. Oral sex and outercourse sit moderate.
Condomless vaginal and anal sex plus shared injection equipment sit high.
And partner infection status plus condom consistency move any act between rungs more than act choice alone. The ladder orients; testing verifies.
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See Tests & Prices →The ladder tiers with reasoning
- Zero (impossible): toilets, pools, hugging, food sharing, mosquitoes, dried blood, casual kissing without sores. Transmission routes don’t exist, so risk is zero by mechanism, not by luck.
- Low (negligible): correct consistent condom use, verified tested mutual monogamy, vaccinated-only exposures. Residual risk from skin-route organisms and human error, small but nonzero.
- Moderate: oral sex (gonorrhea, chlamydia, syphilis, herpes), outercourse (skin-route organisms fully, fluid-borne minimally), condom-protected anal/vaginal sex.
- High: condomless vaginal sex, condomless anal sex (highest per-act HIV risk), shared injection equipment (blood-borne viruses), birth without prophylaxis to infected mothers.
Modifiers that move tiers more than acts do
- Partner infection status dominates: low-tier acts with infected partners transmit efficiently, while high-tier acts with uninfected tested partners transmit nothing.
- Condom consistency demotes acts a full tier when perfect, promotes them when partial (“usually use condoms” protects barely better than never).
- Outbreak presence promotes any contact tier while lesions persist.
- STI coinfection (ulcers, inflammation) amplifies HIV exchange efficiency across tiers.
- Vaccination and PrEP demote personal risk independent of acts.
Using ladders wisely (and their limits)
Prioritize precautions for high-tier acts (condoms non-negotiable, PrEP evaluated, screening frequent).
Allocate testing urgency by tier (high-tier exposures get immediate full protocols including PEP review; moderate-tier patterns get routine scheduling).
Communicate risk honestly with partners using tier language (“anal without condoms is our highest-risk act. Here’s our plan”). And never let ladders reassure: “only oral” still warrants throat swabs, monogamy still needs verification, and testing, not ranking, clears anyone.
See How Do Sexually Transmitted Infections Spread?.
When to seek care
After high-tier exposures (full protocol including PEP review within hours for HIV risk), routinely for moderate-tier patterns on standard schedules, and on schedule regardless of tier self-assessments. Ladders guide behavior, calendars trigger testing.