It has been suggested that the Antonine Plague, which afflicted the Roman Empire during the 2nd century AD, originated in South-East Asia. This is because it bore a close resemblance to similar outbreaks that had swept over China in the years leading up the plague. The Romans were heavily involved in maritime trade in the Indian Ocean, and it's speculated that this is how the plague initially spread.
Whatever the precise source of the disease, there's no doubt that it brought widespread death and disruption to the people of Rome. Something like ten per cent of the Roman population perished during the plague, which most scholars believe to have been either smallpox or measles.
Nowadays, such diseases are easily treated. But in the 2nd century AD, medical knowledge was primitive, to say the least. Treatments for the plague proved completely ineffective, leading many desperate Romans to rely on magic and sacrificial rituals as a way of warding off this deadly disease.
Suffice to say such measures didn't work, and by the time the plague finally subsided, somewhere in the region of five million people lay dead, with the Roman army, many of whose soldiers had come into direct contact with the plague while serving in Asia, especially hard hit.