The question of what caused the Antonine Plague (165-180 AD) and what effects it had has become much more tractable with modern epidemiological and molecular methods. The ancient sources — primarily Galen's clinical accounts — describe symptoms consistent with smallpox: fever, diarrhea, inflammation of the skin, and a rash that appeared on the ninth day.

The demographic impact was severe. Ancient sources describe population losses of up to a quarter in the hardest-hit regions. Modern historians have suggested more conservative estimates, but even a 10-15% mortality rate across the empire would have had profound economic and institutional consequences. The loss of agricultural labor, the disruption of tax collection, the drain on military manpower — all are documented in the historical record.

Marcus Aurelius' reflections in the Meditations, written during the plague years, are shaped by the experience of governing an empire under this kind of stress. The Stoic emphasis on doing one's duty regardless of circumstances was not merely philosophical abstraction for Marcus; it was a practical response to governing during catastrophe.

The Plague of Cyprian (249-262 AD), a second major pandemic, may have been more deadly than the Antonine Plague. Its agent is unknown; the symptoms described by Cyprian of Carthage don't clearly match any known disease. Combined with the military and political crises of the same period (the Crisis of the Third Century), the Plague of Cyprian's demographic impact may have been one of the decisive factors in the Empire's near-collapse.