vfb, you are the knower of things. I do have Gift of Health up in the game, which probably helped her survive so long. But it doesn't heal her because the HP loss is not affliction based.
Would be nice if there was an icon on the unit or a message as to why they died, but not a high priority considering it only affects a small number of units in the overall game. Thanks for the help!
Also, because damage due to disease occurs in the very end of the turn (the same time units usually heal, after all battles etc), you could cast something like Send Lesser Horror to cause a battle in the province she's in, and have her cast Heal and/or personal regeneration. She might be wise enough to target herself with the heal, and the regen should at least keep her alive for a few more turns.