Since then, virologists have been trying to understand how Yersinia pestis, the organism responsible for the infectious disease, can mutate into new strains.

Thankfully, the remains of centuries-old victims have allowed scientists to investigate the disease, as if it were frozen in time. Now, new research published to Molecular Biology and Evolution sheds new light on how the lethal pathogen evolved over five-thousand years.

History's first recorded pandemic, an outbreak called Justinian's Plague, swept across the Byzantine Empire in 541 CE. It took the lives of 25 million people over the course of 225 years. Nearly five centuries later, the same pathogen would kill 60 percent of Europe's population, only this time, we'd know it as the Black Death.

Two years ago, scientists unmasked the bacterium behind Justinian's Plague as the same one that caused the Medieval pandemic.