In 116, Emperor Trajan and his Legions were ramaging deep into Parthian territory.
After the securing access to the Persian Gulf, Trajan stretched the Empire further than it had ever been. He even was able to establish contact with the Kushan Empire in ancient India.
This of course opened huge trade routes that helped pour massive amounts of oriental goods into the Roman trade system, much like the effect the crusades had centuries later.
So say, like in OTL, Trajan, falls ill during the Northern Mesopotamia campaign.
But instead of a heat stroke, he and others in his entourage, contracts the Black plague after coming to contact with said oriental goods.
Then again, like in OTL, he and his entourage sails back to Rome, bringing the disease with them. It then spreads...
What would be the effects of a the Bubonic plauge in ancient Europe?
Would more damage be done?
Would Roman hygiene practices help?
Could civilization collapse?
(FYI This would also mean the Kushan empire is experiencing the Black Plauge as well)