How The Plague Microbe Gave Fleas A Chance

There’s a disease called Far East scarlet-like fever, or Izumi fever. It is caused by a bacterium called Yersinia pseudotuberculosis, which people can catch through contaminated food or water. Its symptoms are usually gentle: fever; stomach pains; nothing worse than a case of appendicitis.

But sometime between 1,500 and 6,400 years ago, this mild-mannered microbe started to change. One particular lineage picked up new genes, while silencing some of its existing ones. It gained the ability to spread via the bites of fleas, and started causing more lethal symptoms. It became what we now call Yersinia pestis—the cause of plague. Three times over, this microbe has ...

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