Lost genes may explain the origin of the Black Death

Pete Seidel/Jack Poland/CDC

About 6,000 years ago, a bacterium underwent a few genetic changes. These allowed it to expand its habitat from the guts of mice to that of fleas. Such changes happen all the time, but in this particular instance the transformation eventually resulted in the Black Death. It wiped out a third of Europe’s population in the 14th century.

Yersinia pestis (YP), which causes the plague, evolved from an ancestral bacteria called Yersinia pseudotuberculosis (YPT). This happened somewhere in China, from where it spread westward causing disease in both animals and humans. In a new study, published in the journal Cell Host & Microbe, researchers at the National Institute of Allergy and Infectious Diseases have retraced the genetic changes that enabled this bug to become one of the most feared microbes.

Making the switch

Infections due to both YPT and YP are classified as “zoonoses,” transmissible from animals (mainly rodents) to other animals, including humans. YPT is transmitted by fecal contamination of food and water. By contrast, the more lethal YP is transmitted via bites of infected fleas.