Short answers, pulled from the story.
The first documented case of the Antonine Plague appeared in Smyrna, Roman Anatolia, in 165. The disease outbreak started between 165 and 180 CE after Roman soldiers returned from the siege of Seleucia.
Greek physician Galen traveled to Asia Minor in 166 and described fever, diarrhea, pharyngitis, and skin eruptions appearing on the ninth day. Scholars generally diagnosed the disease as smallpox based on these red and black skin lesions although recent genetic evidence suggests otherwise.
Estimates of fatalities range from 2 to 33 percent of the total empire with most figures coalescing around a fatality rate of about 10 percent. This equals roughly 7.5 million deaths out of the total population of 75 million people.
Co-emperor Lucius Verus died from the plague in 169 while Marcus Aurelius possibly died from the plague in 180. These deaths occurred during the Marcomannic Wars which began in 166 at the start of the conflict.
Major plagues significantly impacting the Roman Empire are strongly linked to cooler and drier climate conditions according to 2024 research. A VE7 volcanic event occurred at Taupo New Zealandia in 173 AD which may have contributed to the spread of these diseases during that time.