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It killed half the population of Europe in the 14th century, but Yersinia pestis hasn't changed much in 660 years.

The bacteria that caused the Black Death appears to be no more virulent than the variants of the bug still around today, says the international team of researchers that mapped its genetic code. This suggests that environmental factors – including cold and wet weather and poor hygiene – created the "perfect storm" for the pandemic the ravaged medieval Europe. It also shows how far we have come. Today, Yersinia pestis sickens 2,000 people a year in Africa and other countries, but is much less of a threat thanks to antibiotics, better medical care and better sanitation. "The reason we do so well is that conditions are so different," said McMaster University's Hendrik Poinar, an expert in ancient DNA who set out to dig up the molecular secrets of one of the world's most notorious killers.

The mystery: The Black Death claimed 30 to 50 per cent of Europe's population between 1347 and 1351. The bacteria, which causes bubonic plague, is still around and leads to periodic outbreaks in Africa and other countries. The modern version can be effectively treated with antibiotics. But was the ancient version more deadly? Did it have different genes? Is that why it killed 20 to 50 million people in such a short time?

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The detectives: Dr. Poinar was part of the team that looked for answers in the DNA of the medieval bacteria. Their goal: To reconstruct the ancient pathogen from human remains, something that had never been done before, to unearth clues about why it was so deadly. His colleagues included Kirsten Bos of McMaster in Hamilton, Johannes Krause of the University of Tubingen in Germany and Sharon DeWitte of the University of South Carolina.

The victims: Between 1348 and 1350, hundreds of Londoners who were killed by the plague were buried in a cemetery or "plague pit" northeast of the Tower of London. It is hard to picture the horror of the city at the time, Dr. Poinar said. "Imagine just watching people collapse in the streets, the vomiting, the blood, the smell of necrotic tissue.'' The Museum of London excavated 600 skeletons from the site. Dr. Bos and Dr. DeWitte went through them to find 40 teeth that might contain the bacterial DNA they wanted. Four teeth yielded enough Yersinia pestis DNA to study.

The science: The researchers carefully drilled into the pulp of the teeth, which is where they suspected they would find remnants of the blood-borne bacteria. That yielded small pieces and powder. The researchers weeded out all the stuff they didn't want to get the pure extract of Yersinia pestis DNA. A test confirmed it had degraded in a way that is characteristic of ancient DNA. Then they sequenced it and compared its genetic code to the genome of the modern variants.

The results: The medieval and modern variants were strikingly similar; there was nothing in the genome of the Black Death variant to suggest that it was more virulent. "There was no smoking gun," Dr. Poinar said. So why was it so deadly? The Black Death coincided with a period of cooling in Europe, and cold and rainy weather may have left people hungry and vulnerable, he suggested. Hygiene was poor and many people may have been weakened by other infections. As well, it may have been the first time Europeans were exposed to the bacteria. The researchers say they suspected that a different bug caused earlier plagues.

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