Plague mysteriously spread from Europe to Asia thousands of years ago. Scientists now think they know how it was transmitted

For thousands of years, a disease repeatedly struck ancient Eurasia, quickly spreading far and wide. The bite of infected fleas that lived on rats passed on the plague in its most infamous form — the Black Death of the 14th century — to humans, and remains its most common form of transmission today.

During the Bronze Age, however, the plague bacterium, Yersinia pestis, had not yet developed the genetic tool kit that would allow later strains to be spread by fleas. Scientists have been baffled as to how the illness could have persisted at that time.

Now, an international team of researchers has recovered the first ancient Yersinia pestis genome from a nonhuman host — a Bronze Age domesticated sheep that lived around 4,000 years ago in what is now modern-day Russia. The discovery has all