For reference, the last three were monkeypox, COVID, and Ebola.
This is question #48 in the Astral Codex Ten 2023 Prediction Contest. The contest rules and full list of questions are available here. Market will resolve according to Scott Alexander’s judgment, as given through future posts on Astral Codex Ten.
https://en.wikipedia.org/wiki/Public_health_emergency_of_international_concern Base rate is pretty high if this is what he means