If we are to believe the lies of the invaders, that kind of already was the case...
Monsters were sometimes described as infecting humans, sort of like lycans. Ha, werethings.
Undead weren't zombies, but meh, there could be zombies, I guess?
Squirrels and badgers can spread rabies.
Filthy conditions like sewage on the streets can lead to all sorts of disease.
Bubonic plague! Cut the entire continent's population by two-thirds!
This sounds sort of like BT right now...