on a single day. Whether this new epidemic, or reoccurrence of the Antonine plague, impacted the empire outside the city of Rome is unknown. Epidemiology In 166, during the epidemic, the Greek physician and writer Galen travelled from Rome to his home in Asia Minor and returned to Rome in 168, when he was summoned by the two Augusti, the co-emperors Marcus Aurelius and Lucius Verus. He was present at the outbreak among troops stationed at Aquileia in the winter of 168/69. Galen briefly recorded observations and a description of the epidemic in the treatise Methodus Medendi ("Method of Treatment"), and he scattered other references to it among his voluminous writings. He described the plague as "great" and of long duration, and mentioned fever, diarrhea, and pharyngitis as well as a skin eruption, sometimes dry and sometimes pustular, that appeared on the ninth day of the illness. The information that was provided by Galen does not unambiguously identify the nature of the disease, but scholars have generally preferred to diagnose it as smallpox. The historian William H. McNeill asserts that the Antonine Plague and the later Plague of Cyprian (251–c. 270) were outbreaks of two different diseases, one of smallpox and one of measles but 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 involved smallpox. The latter view is bolstered by molecular estimates that place the evolution of measles sometime after 1000 AD. However, Galen's description of the Antonine Plague is not completely consistent with smallpox. Impact Historians differ in their assessment of the impact of the Antonine Plague on Rome. To some, the plague was the beginning of the decline of the Roman Empire. To others it was a minor event, documented by Galen and other writers but little more deadly than other epidemics which frequently ravaged parts of the empire. Estimates of the fatalities from the pandemic range from 2 percent to 33 percent of the Roman Empire's population with deaths between 1.5 million people and 25 million people. Most estimates coalesce around a fatality rate of about 10 percent (7.5 million people) of the total population of the empire with death rates of up to 15 percent in the cities and the army. If the pandemic was