Short answers, pulled from the story.
The Plague of Justinian was caused by the bacterium Yersinia pestis, the same pathogen responsible for the Black Death. Researchers confirmed this in 2013 by detecting Y. pestis DNA in human remains from ancient grave sites dated to the outbreak period.
The most widely supported view is that the plague originated in Central Asia, with modern Yersinia pestis strains in the Tian Shan mountain ranges of Kyrgyzstan, Kazakhstan, and China being most closely related to the Justinian strain. An alternative hypothesis places the origin in sub-Saharan Africa, carried to Egypt by merchants from the Kingdom of Aksum in East Africa.
The mortality rate remains heavily debated. Some scholars estimate the initial outbreak killed up to forty percent of Constantinople's inhabitants and perhaps a quarter of the Eastern Mediterranean's population. Estimates for the full first plague pandemic, which recurred until the mid-eighth century, range from fifteen million to one hundred million deaths across two centuries.
The plague first appeared at the port of Pelusium, near Suez in Egypt, in 541 AD. It spread around the Mediterranean Sea until 544 and persisted in Northern Europe and the Arabian Peninsula until 549, reaching every corner of Justinian's empire by 543.
Yes. According to court historian Procopius, Justinian I contracted the disease in 542 at the height of the epidemic and recovered. The plague is named for him because it struck his empire during his reign.
The plague weakened the Byzantine Empire at the moment its armies were close to reuniting the core of the Western and Eastern Roman Empires. Although the conquest of Italy was completed in 554, the reunification did not hold. In 568, the Lombards invaded northern Italy, defeated the small Byzantine garrison, and established the Kingdom of the Lombards.