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.
"We have good mortality data from outbreaks in nine cities in Europe," Prof Nils Stenseth, from the University of Oslo, told BBC News.
"So we could construct models of the disease dynamics [there]."
He and his colleagues then simulated disease outbreaks in each of these cities, creating three models where the disease was spread by:
- airborne transmission
- fleas and lice that live on humans and their clothes
In seven out of the nine cities studied, the "human parasite model" was a much better match for the pattern of the outbreak.
It mirrored how quickly it spread and how many people it affected.
"The conclusion was very clear," said Prof Stenseth. "The lice model fits best."
"It would be unlikely to spread as fast as it did if it was transmitted by rats.
"It would have to go through this extra loop of the rats, rather than being spread from person to person."
'Stay at home'
Prof Stenseth said the study was primarily of historical interest – using modern understanding of disease to unpick what had happened during one of the most devastating pandemics in human history.
But, he pointed out, "understanding as much as possible about what goes on during an epidemic is always good if you are to reduce mortality [in the future]".
Plague is still endemic in some countries of Asia, Africa and the Americas, where it persists in "reservoirs" of infected rodents.
And, in 2001, a study that decoded the plague genome used a bacterium that had come from a vet in the US who had died in 1992 after a plague-infested cat sneezed on him as he had been trying to rescue it from underneath a house.
"Our study suggests that to prevent future spread hygiene is most important," said Prof Stenseth.
"It also suggests that if you're ill, you shouldn't come into contact with too many people. So if you're sick, stay at home."