Ancient teeth from Justinian's Plague victims recently underwent DNA analysis and results prove that the Black Death (the biggest baddest plague to hit Europe 800 years later) was not caused by some mutated bacterial spawn of Justinian's Plague. The Black Death was an original supervillain.
The ancient teeth, taken from two bodies of a mass Justinian's Plague victim grave in Germany, were able to shed light on the exact bacteria that caused the plague. The dental pulp (the most inner part of a tooth made up of living connective tissue and cells) contained enough DNA to conclude that not only did the plague most likely originate in China, but that the strain of bacteria did not cause or contribute to the Black Death at all, as was previously believed. Justinian's Plague is nothing compared to the elusive Black Death, which killed an estimated 75-200 million people (that's 30 to 60% of Europe's population) beginning in the 14th century.
Compared the the Black Death, Justinian's Plague played a less deadly, but highly crippling role in European history. Caused by Yersinia pestis, the bacteria jumped from rodent to people by way of flea bites. The Plague swept across Europe, northern Africa, and parts of Asia in 542, killing an estimated 30-50 million people. The Roman Emperor at the time, Justinian (hence the Plague's name), tried to get his kingdom back on track but the plague was a major setback and is said to have led to the Empire's downfall. You can't get your Empire back on track when 10,000 people are dying in your capital daily. Justinian even contracted the disease himself but miraculously survived.
The 1,500 year old teeth show that "the plague jumped into humans on several different occasions and has gone on a rampage," says Tom Gilbert, a professor at the Natural History Museum of Denmark. "That shows the jump is not that difficult to make and wasn't a wild fluke."