Given that we're talking about a plague from over a thousand years ago, once it spread to the populations that people were in frequent contact with it basically halts at that point. Everyone has gotten sick and either died or acquired limited immunity. If there is no trade route to spread it, it can't go anywhere on its own. Also I think we can be reasonably sure that if far away merchant arrives at an area where the bodies are literally piling up out side the gates, they won't enter.
It is a 19th and 20th century innovation that viruses can spread so easily.
However, I'm under the impression that the Roman Empire was a lot more linked than we think when we say "they didn't have cars or airplanes".
Sea/river transport has been available since the first monkey climbed on a floating log and it was already widespread, cheap and convenient even 2k years ago.
Human population was only a tiny fraction of 20th century numbers back then. There were no casual travelers from Constantinpole to Buenos Aires at all.
It is a 19th and 20th century innovation that viruses can spread so easily.