A "requirement intended to prevent a specific disease" is something like a vaccination or testing requirement as we've seen with COVID-19, except for a different disease. (Clarification from the comments: The different disease may either be a currently known disease or a newly emergent disease.)
Some requirements that do not count include:
Face masks
Social distancing
Not attending if sick
I'll define "large" as having at least 50 people in attendance, other than third-party staff such as security and venue maintenance. I'll use any published numbers that seem credible or my own approximation otherwise.
"Before 2026" means attending on any date before 2026 in the local time at the event's location, even if the event goes on into 2026.
There may be some edge cases and I will use my own judgment to resolve them. I will be happy to clarify how I will judge other possible cases.
I will not trade in this market.
@StrayClimb Yes, the intent is to include requirements applicable to either an existing disease or any new disease.