From the BBC:
Rats were not to blame for the spread of plague during the Black Death, according to a study.
The rodents and their fleas were thought to have spread a series of outbreaks in 14th-19th Century Europe.
But a team from the universities of Oslo and Ferrara now says the first, the Black Death, can be “largely ascribed to human fleas and body lice”.
The study, in the Proceedings of the National Academy of Science, uses records of its pattern and scale.
The Black Death claimed an estimated 25 million lives, more than a third of Europe’s population, between 1347 and 1351.
More at the link.