I think there were a number of factors. Widespread instability led to people clustering in densely packed cities, where you could live behind walls. This was ob

I think there were a number of factors. Widespread instability led to people clustering in densely packed cities, where you could live behind walls. This was obviously good if you want to avoid being sold into slavery by a roving gang of barbarians, obviously very bad if you want to avoid catching diseases from the ten neighbors you have in your tiny room. Grain production and shipment was pretty centralized, and as it so happened, the disease turned up first in Alexandria, which was the biggest port of Egypt, which was the bread basket of the empire. So with the grain came the rats, then with the rats, the plague, and since Egypt produced a large portion of the grain eaten in the aforementioned overcrowded cities, y. pestis was delivered directly to all the places where it could do the most damage. As other commentators have noted, it's really hard to keep rats out of something they want to be in. Even harder if you live in insanely crowded wooden tenement blocks, as most people did at the time. I also don't think it would have been particularly obvious that the plague was coming from the fleas - the buboes turn up on lymph nodes, not on the flea bites. Worse still, it's also contagious through coughing. So even if they had somehow (and I think it would be impossible) kept the rats out, they would probably still be at risk. The roman empire at this place was pretty dirty, nobody was bathing, dirt floors were the norm, and honestly, y. pestis is a really nasty disease. If you have it today, you're really likely to die. I don't know if it would have been technically possible to stop it given the technology of the day, even if you had all the requisite knowledge. Keeping the rats/fleas out would likely be impossible short of literally burning most cities to the ground - and because of the disease essentially being delivered to all the major population centers, by the time anybody was aware, it would probably be too late anyway.