Scientists have cracked the genetic code of the Black Death, one of history’s worst plagues, and found that its modern day bacterial descendants haven’t changed much over 600 years.

Luckily, we have.

The evolution of society and medicine – and our own bodies – has far outpaced the evolution of that deadly bacterium, scientists said.

The 14th century bug Yersinia pestis is nearly identical to the modern day version of the same germ. There are only a few dozen changes among the more than 4 million building blocks of DNA, according to a study published online Wednesday in the journal Nature.

What that shows is that the Black Death, or plague, was deadly for reasons beyond its DNA, study authors said. It had to do with the circumstances of the world back then.

In its day, the disease killed between 30 million and 50 million people – about 1 of every 3 Europeans. It came at the worst possible time – when the climate was suddenly getting colder, the world was in the midst of a long war and horrible famine, and people were moving into closer quarters where the disease could infect them and spread easily, scientists say. And it was likely the first time this particular disease had struck humans, attacking people without any innate protection.

“It was literally like the four horseman of the apocalypse that rained on Europe,” said study lead author Johannes Krause of the University of Tubingen in Germany. “People literally thought it was the end of the world.”

In devastating the population, it changed the human immune system, basically wiping out people who couldn’t deal with the disease and leaving the stronger to survive, said study co-author Hendrik Poinar of McMaster University in Ontario.

But simple antibiotics today, such as tetracycline, can beat the plague bacteria, which doesn’t seem to have properties that enable other germs to become drug resistant, Poinar said. Plus, changes in medical treatment of the sick, coupled with improved sanitation and economics, put humanity in a far better position. And there’s an immune system protection we mostly have now, Poinar said.

People still get the disease, usually from fleas from rodents or other animals, but not that often. There are around 2,000 cases a year in the world, mostly in rural areas, with a handful of them popping up in remote parts of the United States, according to the Centers for Disease Control and Prevention.

To get the original Black Death DNA, scientists played dentist to dozens of skeletons.

During the epidemic in the 14th century, about 2,500 London area victims of the disease were buried in a special cemetery near the Tower of London. It was excavated in the mid-1980s with 600 individual skeletons moved to the Museum of London, said study co-author Kirsten Bos, also of McMaster University. She then removed 40 of those teeth, drilled into the pulp inside the teeth and got “this dark black powdery type material” which likely was dried blood that included DNA from the bacteria.14th