According to a report on a new study in the Guardian the Black Death was not spread by rats, and there’s even some debate about whether it was plague at all. The evidence against rats (in London at least) is the lack of rate skeletons found. Barney Sloane says that you’d expect to see lots of dead rats in excavations too, but they aren’t there!
Here’s an extract from the article about why rats weren’t to blame:
Mortality continued to rise throughout the bitterly cold winter, when fleas could not have survived, and there is no evidence of enough rats.
Black rat skeletons have been found at 14th-century sites, but not in high enough numbers to make them the plague carriers, he said.
In sites beside the Thames, where most of the city’s rubbish was dumped and rats should have swarmed, and where the sodden ground preserves organic remains excellently, few black rats have been found.