Your Highness! I feared something like this - it appears Ludwig sent assassins to [Player's City]. They poisoned the wells, and your people are sick and suffering. You need to help them fast, and you need units to find the assassins!
Tip: Save researching Monotheism or Medicine until this quest to avoid jumping up to Late Middle Ages technology.
Your Highness! The doctors are working as hard as they can. Slowly, they are managing to cure all poisoned people. And more good news - your units managed to catch the assassins. As I thought - they were sent by Ludwig der Listige.