From the introduction:
Rome 162 AD. The Empire, after reaching the apex of its territorial expansion under the lead of the emperor Traianus in 117 AD., is coming to an end. After a half century of peace, wealth, and development in artistic, urbanistic, cultural, and scientific fields, the specter of war looms once again over Rome, now ruled by emperors Marcus Aurelius and Lucius Verus, brothers in law. The failure of pointless diplomatic politics with the Germanic peoples in the north is about to unleash the barbarian invasions that will so greatly mark Roman history.
Meanwhile to the east, the Parthians, by ravaging the Roman territories of Syria, Cappadocia, and Armenia, force the two emperors to launch a campaign of retaliation. Lucius Verus, driven by Marcus Aurelius, leaves Rome with his armies to recapture these lands and more, up to the enemy’s capital, but at a price. The plague outbreak in the last year of the campaign forces the troops to leave the territories just reconquered. The armies bring the plague home with them, which unleashes a contagion that will be referred to as Antonine Plague.
More than two thousand people will die every day and at least four times that will be newly infected. The Roman army will be decimated, allowing the Germanic tribes to score unprecedented victories. The plague will cripple the Empire for twenty years.