In 166, during the epidemic, the Greek physician and writer Galen traveled from Rome to his home in Asia Minor. He returned to Rome in 168 when summoned by the two Augusti. Galen's observations and description of the epidemic, found in the treatise "Methodus Medendi", is brief. He mentions fever, diarrhea, and inflammation of the pharynx, as well as a skin eruption, sometimes dry and sometimes pustular, appearing on the ninth day of the illness. The information provided by Galen does not clearly define the nature of the disease, but scholars have generally preferred to diagnose it as smallpox.
Historian William McNeill asserts that the Antonine Plague and the later Plague of Cyprian (251-c270) were outbreaks of two different diseases, one of smallpox and one of measles, although not necessarily in that order. The severe devastation to the European population from the two plagues may indicate that people had no previous exposure - or immunity - to either disease. The modern consensus, however, is that both outbreaks were of smallpox.
The epidemic had drastic social and political effects throughout the Roman Empire. Imperial forces moved east under the command of Emperor Verus when the forces of Vologases IV of Parthia attacked Armenia. The Romans' defense of the eastern territories was hampered when large numbers of troops succumbed to the disease. According to the 5th century Spanish writer, Paulus Orosius, many towns and villages in the Italian peninsula and the European provinces lost all their inhabitants. As the disease swept north to the Rhine, it also infected Germanic and Gallic peoples outside the Empire’s borders. For a number of years, these northern groups had pressed south in search of more lands to sustain their growing populations. With their ranks thinned by the epidemic, Roman armies were now unable to push the tribes back. From 167 until his death, Emperor Marcus Aurelius personally commanded legions near the Danube, trying with only partial success to control the advance of Germanic peoples across the river. A major offensive against the Marcomanni was postponed until 169 because of a shortage of Imperial troops.
During the Germanic campaign, Marcus Aurelius also wrote his philosophical work, "Meditations". Passage IX.2 states that even the pestilence around him is less deadly than falsehood, evil behavior, and lack of true understanding. As he lay dying from the disease, Marcus uttered the words "Weep not for me; think rather of the pestilence and the deaths of so many others.