Antonine Plague

In 165 or 166 CE, Roman troops returning from a campaign against the Parthian Empire in Mesopotamia brought a disease home with them. It spread through the cities of the Mediterranean and along the army's supply lines to the Rhine and Danube frontiers. The physician Galen, who was in Rome and later served the emperors, described a fever with a rash of dark sores that began as pustules, and modern scholars think the disease was most likely smallpox, though measles has also been proposed. The outbreak is called the Antonine Plague after Marcus Aurelius Antoninus, one of the emperors who reigned in it.¹⁴

The Roman Empire at the time of Trajan had reached its greatest extent, and it held roughly 60 million people, many in dense cities linked by roads and sea lanes. Armies marched constantly and grain, soldiers and goods moved between provinces. These same networks that made the empire strong made it a perfect highway for infection. The Silk Road and sea trade across the Indian Ocean joined Rome with Persia, India and China, so a pathogen from the east had a route into the empire. Nobody in Rome had immunity to a new disease, and doctors had no cure.¹²

The plague swept back several times until about 180 CE. Ancient writers gave very high numbers, and modern historians estimate that 5 to 10 million people died, perhaps 10 percent of the empire's people, though the figure is much debated. Marcus Aurelius himself died in 180 CE, possibly of the disease, while campaigning on the Danube. Armies were thinned and the emperors had trouble filling the legions, and some historians argue that tax revenue, farm output and population fell in places. Others say the damage was uneven and that Rome recovered, and the question of how deeply the plague hurt the empire is still open.²¹⁵