Not that all are equally semi-founded. At least one turns on a claim of a continuity of knowledge when it is known for a fact that a group lost the knowledge they claimed to have preserved.
I still have to decide where, exactly, new diseases come from. Whether cross-species transmission and mutation, or magic. And ponder how much, if any of this, will get into the story. It is truly annoying when the backstory you start to fudge up to just give a setting grows more luxuriantly than the story itself. Which, if I wanted to, I could fill with lots of quarrelsome people discussing the controversies of history -- and put just about everyone to sleep, no doubt.
At least I have a theory to complicate the heroine's life by inspiring an assassination attempt. And it will prove a useful obstacle when she is trying to get things done