Plague of Justinian (541 - 750 AD)

Justinian I (483 - 565 AD) ruled the Byzantine (aka Eastern Roman) Empire, and reconquered much of the Western Roman Empire before losing it again. Bettmann/Getty Images

The reign of Justinian I, the emperor of the Byzantine Empire in the 6th century, was hampered by an outbreak of bubonic plague. Now known as the Plague of Justinian, this pandemic is thought to have killed between 30 million and 50 million people, perhaps equal to as much as half of the world's population at the time.

The Justinian plague definitely happened, but researchers are still poring over the evidence as to just how bad it was, about 1,500 years ago.

The traditional narrative of this pandemic was that trade largely ceased and the empire was weakened, allowing other civilizations to reconquer previously Byzantine lands in the Middle East, Northern Africa, and parts of Asia. As Justinian was in the process of reuniting the eastern and western halves of the Roman empire when the plague hit, it has even been blamed as the true end of that era.

Ultimately, we know how bad it could have been: half of the world died, the Roman Empire was never united again, and the Dark Ages began.