The Black Death was one of the most devestating diseases of all time so what if it didn't only wipe out Europe but spread to Asia. It starts with most of the European Doctors dying, this keeps the plague spreading. Without the medical research, even more doctors die and the potential investors that could have funded doctors. As it kills and kills, people lose faith in the church and ransac it. Riots breack out and countries fall. The remaining monarchs invest in expeditions to go to the land of the east, where they will find medicine that could possibly cure the plauge.
Unfortuantly the doctors of the East have no cure and the only thing the hundreds of expiditions did was bring hundreds of people with the plague to Asia. The plague spreads, taking all of the asian mice, rats, rabbits and everything else and killing a lot of the asian population, Japan and Russia are the last to fall. Japan is an isolated island but its defenses eventually fail against swarms of fleeing Chinese, Indians and Koreans. Russia is mostly unsurvivable cold but the disease adapts and perserveres until Russia falls.
The only places left are Oceania, Africa and the Americas, what happens next?