Soldiers Bring a Deadly Epidemic Home to the Roman Empire

During the winter of 165 to 166, Roman forces campaigning against the Parthian Empire laid siege to the Mesopotamian city of Seleucia. Ancient writers linked this military operation directly to the eruption of a devastating pestilence that returning legionaries carried westward across the Mediterranean world. While traditional accounts blamed the catastrophe on soldiers who had violated a local sanctuary, the earliest documented infection was recorded in 165 at Smyrna in Roman Anatolia, where the orator Aelius Aristides fell critically ill. Within a single year the contagion reached the imperial capital, initiating an epidemic that would trouble the Roman Empire until approximately 180.

The outbreak struck an empire that had reached its demographic summit, home to an estimated 60 to 75 million inhabitants. Despite the material wealth of the era, living conditions left urban centers especially vulnerable to infectious disease. Roughly one-fifth of the population resided in hundreds of cities, with the capital alone housing around one million people. High mortality meant cities functioned as demographic sinks that relied on steady migration from the countryside to sustain their numbers. Average life expectancy at birth stood in the mid-twenties, and some skeletal studies suggest that ordinary Romans were shorter than the peoples of pre-Roman Italy, reflecting nutritional stresses, crowding, and heavy disease burdens. Once an epidemic started, the empire's roads and sea lanes carried it quickly from city to city.