The plague began shortly after the 1st Jewish Blood Moon lunar eclipses in 162 – 163 AD.
It has been suspected to have been either smallpox or measles, but the true cause remains undetermined. The epidemic may have claimed the life of Roman emperor Lucius Verus, who died in 169 and was the co-regent of Marcus Aurelius Antoninus, whose family name, Antoninus, was given to the epidemic. The disease broke out again nine years later, according to the Roman historian Dio Cassius, and caused up to 2,000 deaths a day in Rome, one quarter of those infected. Total deaths have been estimated at five million. The disease killed as much as one-third of the population in some areas and devastated 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 the legions along the Rhine. Eutropius asserts that a large population died throughout the Empire.