Marcus Aurelius. Photo: © DEA Picture Library/Art Resource, NY. The year was 166 C.E., and the Roman Empire was at the zenith of its power. The triumphant Roman legions, under the command of Emperor Lucius Verrus, returned to Rome victorious after having defeated their Parthian enemies on the eastern border of the Roman Empire. As they marched west toward Rome, they carried with them more than the spoils of plundered Parthian temples; they also carried an epidemic that would ravage the Roman Empire over the course of the next two decades, an event that would inexorably alter the landscape of...