Plague bacterium genome: All of the DNA (the genome) of the bacterium Yersinia pestis that causes the plague, consisting of four rings of DNA -- a large circular chromosome made up of 4,653,728 bases of DNA and three much smaller rings, or plasmids -- and including about 150 genes made inactive by mutation, a process that affects genes the organism no longer needs. Since some of these inactive genes are known in other bacteria to be involved in colonizing the intestinal tract, it has been inferred that the plague bacterium's ancestor once lived in the gut and then acquired genes from other bacteria that let it move from the gut to the blood, and also conferred upon it the ability to colonize fleas. Thus armed, the bacterium morphed from a minor cause of gastroenteritis to a major scourge.

The sequencing of the plague genome was carried out at the Sanger Centre in England and completed in 2001.