the antonine plague of 165 to 180 ad, also known as the plague of galen (from the name of the greek physician living in the roman empire who described it), was an ancient pandemic 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 died in 169 c.e and was the co-regent of marcus aurelius antoninus, whose family name, antoninus, has become associated with the epidemic. the disease broke out again nine years later, according to the roman historian dio cassius (155–235), causing up to 2,000 deaths a day in rome, one quarter of those who were affected, giving the disease a mortality rate of about 25%. the total deaths have been estimated at five million, and the disease killed as much as one-third of the population in some areas and devastated the roman army.