During the reign of Marcus Aurelius, the Roman Empire was struck by a long and destructive epidemic. It began in Mesopotamia in late AD 165 or early AD 166 during Verus' Parthian campaign, and quickly spread to Rome. It lasted at least until the death of Marcus Aurelius in AD 180 and likely into the early part of Commodus' reign. Its victims were "innumerable". Galen had first-hand knowledge of the disease. He was in Rome when the plague reached the city in AD 166. He was also present during an outbreak among troops stationed at Aquileia during the winter of AD 168-169. His references to the plague are scattered and brief but enough information is available to firmly identify the plague as smallpox. His description of the exanthema is fairly typical of the smallpox rash, particularly in the hemorrhagic phase of the disease.