The thing about plagues is that they tend to catch everyone off guard. That was certainly the case when I triggered a plague age in Rome in the early common era. But since these undesirable ages are triggered by the accumulation of chaos and unrest in the first place, it wasn’t as though my Roman empire was in great shape before the coughing started.
I’d been fighting off barbarians for so long, I inadvertently triggered a chaos event: a rebellion, spawning pikemen and cavalry beside every regional capital. Thus I had to commit even further to channelling all my resources into a military offensive on several fronts, which raised unrest even more, until the plague doctors arrived and my population dropped by a third. Brutal? Yes.
Admirable? That too.