How Risky Is One Unprotected Encounter, Really?
This is a genuinely hard question to answer with one number, because per-encounter transmission risk varies enormously depending on which infection, which specific sexual activity, and whether the other partner actually has the infection in the first place. Here's an honest attempt at context rather than a false sense of precision.
Why there's no single answer
Per-act transmission probabilities differ by orders of magnitude between infections — HIV transmission risk from a single act of vaginal sex with a known HIV-positive partner is estimated in the range of roughly 1 in 1,000 to 1 in 2,000 for a receptive partner (higher for receptive anal sex, lower for insertive), while gonorrhea and chlamydia transmit far more readily per encounter. All of these numbers also assume the partner definitely has the infection, which — statistically, for a random partner — is far from certain and depends heavily on local prevalence and that specific person's history.
The more useful framing
Rather than fixating on a precise probability that can't really be calculated for your specific situation, the more useful approach is: one unprotected encounter is genuinely not negligible risk, it's not guaranteed transmission either, and the productive next step is testing at the appropriate window (not immediately — see how soon to test after unprotected sex) rather than trying to calculate odds that can't be known precisely anyway.
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