According to some historians, the success ancient Rome's "Five Good Emperors" had in centralizing the empire's administration, while undoubtedly bringing huge benefits, sowed the seeds for later problems. After all, as so many Roman emperors proved, from Caligula and Nero to Commodus, the empire's approach to governance was predicated on the ruler's ability. When incompetent or insane emperors came to power, the whole edifice came tumbling down. Moreover, the success of the emperors ironically brought about the worst plague in Rome's epic history. Due to constant warfare on the borders and attempts to defend positions against various groups, Roman soldiers came into contact with foreign diseases, and they unwittingly brought them home when campaigns ended. This culminated around 165 CE, when an unidentified disease brought the empire to its knees and afflicted an untold number of individuals, one of whom may have been Lucius Verus, the co-emperor of Rome alongside Marcus Aurelius.
In addition to the enormous number of casualties, which has been estimated at upwards of 5 million people, the pandemic disrupted Roman trade to the east, affected societies culturally across Europe, and compelled physicians like Galen to study the symptoms in an effort to figure out not only what the disease was, but any potential cures. Of course, that was a tall task for ancient doctors with relatively primitive technology, and even today people continue to debate what the disease was and where it came from, with theories ranging from a smallpox outbreak in China, or possibly measles.
