For 800 years, we've blamed black rats for the waves of plague that killed millions in Europe. But new data is pointing to the climate in Asia and its impact on the great Asian gerbil.

How? Well, the great gerbil is a natural host of plague bacterium, Yersinia pestis. It was transmitted to humans by fleas and brought to Europe by traders. Once there, it infected rats, which then spread it.

We believed the rats were behind subsequent waves of plague there, but the rodents are still in Europe - so why is there no plague?

Researchers theorized that these rats were not a reservoir for plague and that each individual wave came from Asia. They began looking at climate records in tree rings and found the incidence of plague correlated not with climate changes in Europe but with Asia.

When the weather was dry in Asia, the gerbil population crashed. Plague-carrying fleas then sought other hosts, such as traders and their pack animals. Scientists discovered a 15-year lag from the time of dry Asian weather before plague made it to Europe. They proposed three phases of this lag.

The first phase, lasting one to two years, is when fleas sought other hosts. The second phase is the longest at ten to twelve years - the time required to travel from Central Asia to the Black Sea in Southeastern Europe. The third phase involved the infection of rats that stowed away on ships, spreading plague from harbor to harbor.

To definitively prove this, researchers need DNA sequencing of the European plagues. Genetically distinct bacterium would support this new theory that each wave was introduced from Asia. If instead they're similar, then there must be a reservoir for plague in Europe and other explanations for the pandemics must be considered.

As for those cute gerbils in the pet store, don't worry. They're safe.