'Gerbils replace rats' as main cause of Black Death
bbc.com
bbc.com
It's hard looking that far back in time ... but we've got some amazing techniques now. We've found enough DNA in exhumed victims of not only the Black Death, but the 6th Century Plague of Justinian, 1,500 years ago (DNA is hardy stuff) to confirm they were distinct strains of Yersinia pestis.
From what animal reservoirs they jumped from, though, pretty much has to be speculative. But it is interesting they're smelling a new "rat" ^_^.