Why did the Antonine Plague start?

The Antonine Plague (smallpox and measles) most likely emerged in China shortly before 166 CE spreading westward along the Silk Road and by trading ships headed for Rome. Sometime between late 165 to early 166 CE, the Roman military came into contact with the disease in western Asia and brought it back to Rome.

The epidemic most likely emerged in China shortly before 166 CE spreading westward along the Silk Road and by trading ships headed for Rome. Sometime between late 165 to early 166 CE, the Roman military came into contact with the disease during the siege of Seleucia (a major city on the Tigris River). One factor in this may be the Antonine Plague, a mysterious epidemic that the army brought back from West Asia which ravaged the empire from roughly 165-190 CE. The Greek physician Galen described symptoms common to smallpox, and the loss of life was incredible.