Gerbils ?

The latest theory about the Black death is that it was spread in some area by....gerbils.

Humans caught the plague from rats, but researchers say it was actually gerbils that were the original breeding ground for the disease
The fleas made their way to Europe in various stages. Some would have arrived via foxes that had eaten the gerbils, which then passed them onto rats, which passed them to humans.In other cases humans may have caught the disease from camels, the fleas having jumped to them from the gerbils.

Read more: http://www.dailymail.co.uk/news/article-2966392/Origins-plague-ratted-actually-GERBILS-main-cause-Black-Death.html#ixzz3SfeNNjmg Follow us: @MailOnline on Twitter | DailyMail on Facebook

in other words, the original source was gerbils but the spread was via other animals.

And in the US, the plague is found in the lowly prairie dog, and every year there are a few cases in the US...docs on the Navajo reservation are especially alert to cases popping up. 

the source is rock squirrels and prairie dogs.