The Antonine Plague on 165 to 180 CE, also known as the Plague of Galen (after the Greek physician and writer who recorder eyewitness observations), is a pandemic, either of smallpox or measles, brought back to the Roman Empire by troops returning from campaigns in the Near East.

Scholars have suspected it to have been either smallpox or measles,[ but the true cause remains undetermined

The epidemic may have claimed the life of a Roman emperor— Lucius Verus, who dies in 169 and is the co-regent of Marcus Aurelius Antoninus, whose family name, Antoninus, will become associated with the epidemic.

The disease breaks out again nine years later, according to the Roman historian Dio Cassius, and causes up to two thousand deaths a day at Rome, one quarter of those infected, giving the disease a mortality rate of about twenty-five percent.

The total deaths have been estimated at five million, and the disease kills as much as one-third of the population in some areas and devastates the Roman army.

Ancient sources agree that the epidemic appeared first during the Roman siege of Seleucia in the winter of 165–166.

Ammianus Marcellinus reports that the plague spread to Gaul and to the legions along the Rhine

Eutropius asserts that a large population died throughout the Empire.

Rafe de Crespigny will speculate hat the plague may have also broken out in Eastern Han China before 166, given notices of plagues in Chinese records.

The plague affects Roman culture and literature, and may have severely affected Indo-Roman trade relations in the Indian Ocean.