Once the Undead were truly dead, the work of cleaning them up to prevent the spread of plagues or even the Zombie Virus to the surrounding ecosystem began.
The worst thing that you could do, in the minds of the soldiers, was to fight a Zombie in running water. The moment that you began to damage it, the undead creature would leak infected fluids into the water, and there was no telling what life form downstream might ingest that in enough concentration that it would turn them and begin an outbreak.
Towns along the rivers had detection magic to ensure that nothing of the sort, or any other magical toxin, would make its way through their territories, but a zombie monster could spread the plague just as easily as a zombie farmer or dockworker.
