Don't blame rats for the Black Death. Scientists now think it was Asian giant gerbils that carried the fleas that spread the plague.
From CBC News:
The University of Oslo researchers, working with Swiss government scientists, say a "pulse" of strains arrived sporadically from Asia. They posit the Yersinia pestis bacterium was likely carried over the Silk Road via fleas on the giant gerbils during intermittent warm spells.
The fleas could have then transmitted the disease to humans.
Great gerbils, which are found in many parts of Central Asia, can grow to adult sizes of about 15-20 centimetres in body length.