Different from the Black Death 
 
 The strain of Yersinia pestis germ which caused the Plague of Justinian was different from the strain that triggered the Black Death in the 14th century, killing an estimated 30 million Europeans, they found. The evidence confirms, as expected, the role of rats as the germ's "reservoir" or natural source, according to a paper published in the journal The Lancet Infectious Diseases. [6 Superbugs to Watch Out For] 
 
 "Plague remains a real threat in some places of the world, like Madagascar, which has more human plague cases than any other country — hundreds to over a thousand cases a year," said Dr. Dave Wagner, co-first-author on the study and an associate professor in the Center for Microbial Genetics and Genomics at Northern Arizona University. Using DNA teased from 1,500-year-old teeth of plague victims buried in Germany, scientists have reconstructed the genetic profile of the killer and say its ability to mutate is a warning for people today. Assembling their genomes is a feat, for they are the DNA codes of the oldest pathogens obtained to date. The first of three plague pandemics to have ravaged humanity, it killed between 25 and 100 million people across Asia, North Africa and Europe.
– 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.