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The Debunker: Did Rats Cause the Black Death?

by Ken Jennings

On May 11, 1963, a Los Angeles couple named Ron and Phyllis Patterson held a weekend-long radio station fundraiser they called a "Renaissance Pleasure Faire," giving birth to a whole new entertainment industry based on corsets, meat pies, and stilted faux-Shakespearean English. This month, to celebrate the 55th anniversary of Ye Olde Renaissance Festival, we've asked Jeopardy!'s Ken Jennings to drop by and give us the scoop on what really went on during the Middle Ages. He's going to get medieval on your ass…umptions.

The Debunker: Did Rats Cause the Black Death?

I'm not one of those weird guys who always had a rat or ferret running up and down in his arm in college. I'm not crazy about rodents. It would give me great pleasure to blame the "Black Death" of the Middle Ages, which may have killed over one hundred million people, on the icky little vermin. That's the way the epidemic is usually taught in history classes, after all. But the science of bubonic plague is actually not a settled matter at all—and the latest research is actually on the side of exone-rat-ion.

The Debunker

I'm not talking about the gerbils, by the way. Did you see those new stories in 2015, the ones that pinned the blame on gerbils as the real carriers of the fleas that spread the plague? If you read the actual paper that spawned all the clickbait headlines, you'll find that gerbils are not directly implicated by the new research. They're just one of several Eurasian mammals mentioned in the study (alongside ground squirrels and Altai marmots) as possible vectors for the disease, which, the authors believe, may have been introduced to Europe in multiple waves, and not just from a single outbreak.

But a new study from January of this year has found a new culprit. A team of biologists at the University of Oslo was researching one of the weirdest things about the Black Death: that it spread so much faster than other bubonic plague outbreaks in the historical record. The scientists ran computer models for two modes of transmission: one via fleas on rats, and one based on human parasites like lice. The human-parasite model actually fit the mortality data better in seven of the nine cities they studied. More work with bigger data sets is required, but many scientists now believe that the traditional explanation is wrong, and that the Black Death was spread directly by fleas and lice hopping from person to person, without a stage where fleas drank infected rat (or gerbil!) blood at all. Maybe Europe didn't need exterminators and rat-traps to stop the plague—just more baths and better "manscaping."

Quick Quiz: What political activist and Band Aid founder was the lead singer of the Boomtown Rats?

Ken Jennings is the author of eleven books, most recently his Junior Genius Guides, Because I Said So!, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at or on Twitter as @KenJennings.