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It only took one visit for plague – the Black Death – to get a foothold in Europe and kill millions of people over the next 400 years, researchers reported Wednesday.

A team of scientists dug up ancient plague pits from across Europe and then dug into the DNA of some of the victims to build a genetic map of plague.

Red arrows indicate Branch 1 of plague cycling through Europe during the 14th century, eastward travel out of Europe after the Black Death, and global dissemination from China during the third plague pandemic Maria Spyrou, Max-Planck Institut et al / Cell Host & Microbe

They found evidence the killer Yersinia pestis bacteria came to Europejust once at the beginning of the medieval period. It hung around for the next 400 years, possibly spreading back eastwards into Asia.

The findings, published in the journal Cell Host & Microbe, counter other theories about how plague came back time and again to claim as much as 60 percent of the population of Europe. Previous theories had held the infection, carried by fleas, was re-introduced several times by travelers from Asia.

“Our results provide support for a single entry of Y. pestis in Europe during the Black Death, (and) a wave of plague that traveled toward Asia to later become the source population for contemporary worldwide epidemics,” Maria Spyrou of the Max Planck Institute for the Science of Human History in German and colleagues wrote in their report.

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