The black death that ran rampant through Europe between 1346 and 1353 may not have been caused by rats, as usually suspected, but by the innocent-looking gerbil.

The black death was one of the most devastating epidemics in human history, taking the lives of between 75 and 300 million people. Also known as the bubonic plague, the disease is believed to have originated around China, and traveled up the Silk Road to Europe.

Black rats are usually implicated in the process, hosting fleas that spread the disease. However, new climate research examining conditions in Europe during the 14th century appears to contradict that idea.

"For this, you would need warm summers, with not too much precipitation... And we have looked at the broad spectrum of climatic indices, and there is no relationship between the appearance of plague and the weather," Nils Christian Stenseth, one of the authors of the new study, said.

Tree rings were examined, and compared with 7,711 records of plague outbreaks, to see if these appearances of the disease coincided with environmental conditions that would have been beneficial for rat populations to thrive. Researchers found there were no correlations between plague outbreaks and rat populations.

However, wet springs followed by hot summers cause gerbil and flea populations to boom. When these conditions were seen in historical records in Asia, the disease spread a few years later in Europe. Trade between Asia and Europe was flourishing at the time, providing a way for the disease to spread among humans between continents.  

Starting with the first series of epidemics, the disease came back in several outbreaks over the next four centuries. Millions of people died during these later appearances of the illness.

Further examination of skeletons of plague victims could reveal the degree of genetic diversity in the bacteria. If a large amount of variation is found within the genetic structure, the finding would lend additional support to the theory of climatic changes in Central Asia leading to spread of the bacteria through gerbils.

Although the black death has not been a major health concern in Europe since the 19th century, the disease still infects some victims in other parts of the globe. The World Health Organization estimates that nearly 800 people were infected with the disease in 2013, of whom 126 died.

Agriculture development in eastern Africa may be placing human populations there at risk of future outbreaks of the disease, earlier research determined.

Study of the role gerbils may have played in the black death in Europe was profiled in the journal Proceedings of the National Academy of Sciences.