Final answer:
The pandemic known as Justinian's Plague was most likely caused by the bacterium Yersinia pestis, leading to a massive outbreak of bubonic plague.
Step-by-step explanation:
The pandemic known as Justinian's Plague was likely caused by a bacterial infection. Specifically, the outbreak of the Plague of Justinian from 541 to 750 CE is believed to have been caused by the bacterium Yersinia pestis, which is the pathogen responsible for bubonic plague. Historical records suggest that this pandemic eliminated one-quarter to one-half of the human population in the eastern Mediterranean region, and it is notable that the European population dropped by about 50 percent during this time. This pandemic had profound impacts on society and is considered one of the first recorded pandemics in history.