This is Planet Money from NPR.

We're going to start today's show on a bustling fall day in Sicily,

in a port town called Messina. The year is 1347, and ships are coming into the harbor

from all around the Mediterranean. They're loaded up with spices and silk

from Asia, summer filled with grain. And in addition to the spices and the

silk and the grain, these ships are also carrying a very different kind of cargo,

a kind of stow-away.

That would be the rat, and in particular the black rat, or ratus,

ratus, as it was called scientifically, and it's actually rather cute.

Frank's Noden is a medical historian at Yale, and Frank says that on that cute

little ratus, ratus, is another stow-away, which is the flea.

And inside the gut of this little flea is a bacteria called

Yersinia Pestus, a bacteria that is in the process of unleashing a global catastrophe.

Yersinia Pestus is really one of the most awful bacteria that you can

imagine ever having to face.

Awful because Yersinia Pestus is what causes the bubonic plague,

and it starts right there inside the gut of the flea,

where it immediately reproduces like crazy.

And it does so in such profusion that it clogs the digestive system of the

poor flea, and the flea begins to die of starvation.

And so it becomes more and more compelled to bite.

Bite the rats, who then get their rat version of this awful disease.

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