kinda wonder about the speed of the spread. I suspect it's not guys bringing it back from a great distance, but simply the climate improves in a broad area after a drought; the grass grows; the animal vector thrives; surges out beyond its normal holes in the ground, and infects humans ~ through the mechanism of the flea!
The animals were there all along during a moderate climate, but they weren't doing well so they didn't get to spread the disease.
People would look around for a cause and notice that a returning army, still living outside, came down with the plague before townsmen.