Galen

The Plague of Galen, from 165-180 CE, bears the name of the Greek physician who historically described it. This plague was the first known pandemic to impact the Roman Empire, interrupting the last years of co-emperor Marcus Aurelius' reign. This period was also the tail-end of the Pax Romana, the high-point of Roman territorial control and influence. But while the Empire may have been prospering economically, the people were unhealthy, and overcrowded cities with dangerously unsanitary conditions created perfect conditions for the spread of disease. Epidemics of infectious diseases were common in the century leading up to the plague, and the increased connectivity of the Empire made it easy for diseases to travel. Even prior to the plague, death rates exceeded birth rates, and more than half of all children died before reaching adulthood. The unhealthy conditions of the population can be inferred from studying their remains, which belay signs of disease, malnutrition and stunted growth.

According to the Romans, the plague was brought into the Roman Empire by soldiers returning from the sack of the Parthian city of Seleucia in 165 CE. It was also believed that the plague was sent as a punishment for the Roman army's violation of Seleucia's main temple. The first documented case of the plague was in the Ionian city of Smyrna that same year, where the orator Aelius Aristides almost died from the disease. From the east the plague spread westward, reaching Rome in 166 and nearly every corner of the empire by 172, ravaging cities and decimating the Roman army.

In 166, the Greek physician and writer Galen travelled from Rome to his home in Asia Minor, then returned to Rome in 168 when he was summoned back by the two Augusti. 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"). 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. Galen's observations do not unambiguously identify the nature of the disease, but scholars have generally preferred to diagnose it as smallpox despite his description not being completely consistent with smallpox symptoms. Measles has also been suggested, but recently molecular findings place the evolution of measles sometimes after 1000 CE.

When the first plague arrived in 166, the Marcomannic Wars with the Germanic tribes invading Roman territory had just begun. The impact of the plague forced the Augusti to recruit and train additional soldiers from among "gladiators, slaves, and bandits." The need to train recruits delayed Roman war efforts by two years, and then in 169 CE, co-emperor Lucius Verus died of an unconfirmed illness, possibly food poisoning or the plague. After deifying Verus at his funeral, Marcus Aurelius launched an attack against the Germanic tribes. By 171 the Roman army had driven the invaders out of Roman territory. The war would continue sporadically until 180 when Marcus Aurelius also died, possibly of the plague. Lucius Verus and Marcus Aurelius had both been adopted by the previous emperor, Antoninus Pius. As a result, their family name, Antoninus, has become associated with the pandemic, also known as the Antonine Plague.

The plague endured until about 180 CE, and according to the contemporary Roman historian Cassius Dio, the disease broke out again nine years later in 189, causing up to 2,000 deaths a day in the city of Rome, one quarter of those who were affected. The total death count has been estimated at 5–10 million, roughly 10 percent of the population of the empire. The severe devastation to the population from the plague may indicate that people had no previous exposure to the disease, which then granted immunity to survivors.

Historians differ in their opinions of the impact of the plague on the empire in the increasingly troubled eras after its appearance. 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 2-33% 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% (7.5 million people) of the total population of the empire with death rates of up to 15% in the cities and the army. If the pandemic was indeed smallpox, the number who died would probably have been about 25 percent of those infected as the survival rate from smallpox is often around 75%, or three out of four people infected.

The plague also spread beyond the borders of the Roman Empire, possibly reaching or even originating near China. A possible contact with Han China occurred at the beginnig of the pandemic, in 166 CE, when a Roman traveller visited the Han court, claiming to be an ambassador representing a certain Andun (Chinese: 安 敦), ruler of Daqin, who can be identified either with Marcus or his predecessor Antoninus. In addition to Republican-era Roman glasswares found at Guangzhou along the South China Sea, Roman golden medallions made during the reign of Antoninus and perhaps even Marcus have been found at Óc Eo, Vietnam, then part of the Kingdom of Funan near the Chinese province of Jiaozhi (in northern Vietnam). This may have been the port city of Kattigara, described by Ptolemy (c. 150) as being visited by a Greek sailor named Alexander and lying beyond the Golden Chersonese (i.e. Malay Peninsula). Roman coins from the reigns of Tiberius to Aurelian have been found in Xi'an, China (site of the Han capital Chang'an), although the far greater amount of Roman coins in India suggests the Roman maritime trade for purchasing Chinese silk was centred there, not in China or even the overland Silk Road running through Persia.

In the view of some historians, the plagues afflicting the Eastern Han empire of China during the reigns of Emperor Huan of Han (r. 146–168 CE) and Emperor Ling of Han (r. 168–189 CE), which struck in 151, 161, 171, 173, 179, 182, and 185, were perhaps connected to the plague in Rome. While Roman contact with the Han Chinese court in 166 may have started a new era of Roman–Far East trade, it may also have enabled the spread deadly illnesses. The spread of the plague disease caused irreparable damage to the Roman maritime trade in the Indian Ocean and significantly decreased Roman commercial activity in Southeast Asia. However, Roman maritime trade into the Indian Ocean, particularly in the silk and spice trades, certainly did not cease but continued until the loss of Egypt to the Muslim Rashidun Caliphate.

Some historians have hypothesized that the plague resulted in a surge in the popularity of the cult of Asclepius, the god of medicine; epigraphic evidence, however, suggests no such increase of the cult's popularity occurred.