Antonine Plague (jan 1, 165 – jan 1, 180)

Description:

ancient pandemic brought back to the Roman Empire by troops returning from campaigns in the Near East. Scholars have suspected it to have been either smallpox[1] or measles,[2][3] but the true cause remains undetermined.

The total deaths have been estimated at 5 million, and the disease killed as much as one-third of the population in some areas and devastated the Roman army.

https://en.wikipedia.org/wiki/Antonine_Plague