From Science Daily: Scientists Discover Why Plague Is So Lethal.

Bacteria that cause the bubonic plague may be more virulent than their close relatives because of a single genetic mutation, according to research published in the May issue of the journal Microbiology.

"The plague bacterium Yersinia pestis needs calcium in order to grow at body temperature. When there is no calcium available, it produces a large amount of an amino acid called aspartic acid," said Professor Brubaker from the University of Chicago, USA. "We found that this is because Y. pestis is missing an important enzyme."

Bubonic plague has killed over 200 million people during the course of history and is thus the most devastating acute infectious disease known to man. Despite this, we are still uncertain about the molecular basis of its extraordinary virulence.

"Y. pestis evolved from its ancestor Y. pseudotuberculosis within the last 20,000 years, suggesting its high lethality reflects [continue]

As some of you were discussing Y pestis in this comment thread, I thought you might be interested.