New Scientist: Researchers have failed to find traces of the DNA of Yersinia pestis, the bacterium carried by rat fleas thought to have caused the Plague, on 121 teeth from 66 skeletons from "plague pits" in England, Denmark, and France. The researchers, from Oxford University and Bart's Hospital and the London Hospital, have been studying the skeletons' teeth because each tooth's enclosed contents may not disintegrate the same way as other body tissues. Each tooth has its own blood supply, which can also mean that high levels of plague bacteria could leave DNA traces inside the tooth.

Earlier French research, which found the Yersinia bacterium in DNA from teeth in southern France, may have been flawed. "This can't prove that the Black Death wasn't Yersinia pestis, just that we couldn't reproduce the results of the French team," said Dr. Michael Prentice, a medical microbiologist from Bart's Hospital, when speaking with BBC News. "Ancient DNA is a very difficult area -- it may be that these techniques will never be enough to provide an answer."