When Antoninus died in 161 A.D., Marcus Aurelius took the position of emperor. He had to spend most of his time in the field at the front of his armies. One of his armies brought back the most virulent plague of the Roman era, which spread throughout the empire. Once Marcus Aurelius died in 180 A.D., the empire was once again under unease. http://www.military-history.org/articles/thinkers-at-war-marcus-aurelius.htm