The Plague of Justinian and the Black Death arose from separate bacteria strains, researchers say—and that's not a good thing, because if distinct plagues have ravaged the human population before, they could come up again, LiveScience reports. A group of researchers came to this conclusion by digging up two victims of the Black Death in Germany and studying DNA fragments in their teeth. They reconstructed the disease's genome and compared it to more than a hundred contemporary strains of plague, but no luck—the Black Death was an evolutionary dead end.
This "generates new questions," says the study's top author. "For example, why did this [Justinian] pandemic, which killed somewhere between 50 and 100 million people, die out?" Maybe people built up an immunity to the bacteria, AFP notes, or natural climate variation could have stopped the germs from spreading. Either way, plague remains a threat in some countries, like Madagascar, where people live in closer proximity to flea-carrying rodents. Even the US has a few bubonic plague infections each year. "Fortunately we now have antibiotics that could be used to effectively treat plague, which lessens the chances of another large scale human pandemic," says a co-author of the study. (Read more plague stories.)