The Black Death

"The Black Death claimed the lives of one-third of Europeans in just five years from 1348 to 1353. Until recently, it was not certain whether the bacterium Yersinia pestis -- known to cause the plague today -- was responsible for that most deadly outbreak of disease ever. Now, the University of Tübingen's Institute of Scientific Archaeology and McMaster University in Canada have been able to confirm that Yersinia pestis was behind the great plague."

But don't worry too much. They believe the particularly deadly strain of Y. pestis that caused the plague in the middle ages is extinct. Modern plague is much milder, and treatable with antibiotics.

How'd they get DNA samples of plague from the middle ages? In the creepiest possible way: they dug up old bodies in a London plague cemetery.