Epidemiology
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; he was present at the outbreak among troops stationed at Aquileia in the winter of 168/69. Galen's observations and description of the epidemic in the treatise Methodus Medendi is brief, and his other references to it are scattered among his voluminous writings. He describes the plague as "great" and of long duration and 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–ca.270) 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 to either disease, which brought immunity to survivors. Other historians believe that both outbreaks were of smallpox. This latter view seems more likely to be correct given that molecular estimates place the evolution of measles sometime after 500 AD.
Read more about this topic: Antonine Plague