AD 166 was a year during the Roman Empire, notable for its events and developments. It was part of the reign of Emperor Marcus Aurelius, who ruled from 161 to 180 AD. This period was marked by military conflicts, particularly against the Parthian Empire in the east, as well as challenges from various tribes along the northern borders. In AD 166, the Roman Empire faced a significant outbreak of the Antonine Plague, which affected the population and military strength. This pandemic is believed to have been caused by either smallpox or measles, leading to widespread illness and contributing to the empire's struggles during this time.