How did the plague weaken the Roman Empire under Marcus Aurelius?

During the reign of Marcus Aurelius, a plague brought back by returning armies from the Parthian War (162–166) significantly weakened the Roman Empire. This epidemic, which persisted for many years, caused severe casualties—with estimates suggesting as many as 2,000 deaths per day at its height—and contributed to a notable decline in the empire's population.12

The plague functioned as a catalyst for instability by fostering a sense of demoralization among a populace that had grown accustomed to the perceived immutability and security of the Roman Empire. This psychological impact, combined with the simultaneous pressure of Germanic invasions, exposed the military and financial precariousness of the state. The resulting strain forced the government to adopt desperate measures, such as auctioning imperial property to fund the military and seeking unconventional ways to fill the ranks of depleted legions. Ultimately, the plague undermined the stability of the era, complicating the efforts of Marcus Aurelius to maintain the empire's defenses and internal order.1