For hundreds of years, the arrival of the bubonic plague in Europe in
the mid-14th century has been blamed on rats. However, a new study
released this week has put a different rodent under suspicion: gerbils. 
Scientists at the University of Oslo in Norway have claimed that that
the deadly disease was repeatedly brought to Europe from Asia via trade
routes such as the Silk Road, and with gerbils, not rats as the
carriers.
"If we're right," study co-author Nils Christian Stenseth told the BBC, "we'll have to rewrite that part of history."
The study examined over 7,700 tree ring records that revealed climate
information about Europe during that period. They found that outbreaks
in Europe occurred approximately 15 years after a spell of wet weather
and warmer temperatures in Asia, which would have bolstered the gerbil
and flea populations. By contrast, the timing of the outbreaks in Europe
did not appear to coincide with any weather pattern on the continent.
Previously, black rats who had stowed away on merchant ships were
thought to have enabled the plague to establish itself in Europe, with
fleas spreading the bacteria by jumping from infected rat to
humans. However, Stenseth told NPR
that according to that theory, rats and their fleas should still be
spreading plague in European cities today, when in fact, it has been
nearly 300 years since a major outbreak.
The study theorizes that fleas carrying the plague bacterium jumped
from gerbils to pack animals and humans, some of whom were traders who
brought the disease to Europe. The next step is to analyze the DNA of
plague bacteria, which can be found in the skeletons of its victims. If
the Oslo theory is correct, the bacterial DNA would vary widely across
each outbreak, as the disease would have likely changed each time it
entered Europe.
Historians have estimated that the first outbreak of plague in
medieval Europe, between 1346 and 1353, killed between 75 million and
200 million people, at least a third of the continent's population.
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