Contagious cancers were thought to be exceedingly rare—after all, researchers only knew of three kinds in the world. But now, there’s a fourth. And it’s raising some big questions about scientists’ understanding of cancer.

This week, scientists report finding a new type of transmissible tumor in Tasmanian devils, the famous marsupials of the Australian island state. It’s the second type of infectious cancer seen in devils and the fourth type overall. The finding, published in the Proceedings of the National Academy of Sciences, has left the study authors questioning whether infectious cancer cells are more common than expected or if there’s something about Tasmanian devils that makes them uniquely susceptible to catching deadly tumors—or maybe both.

“Regardless,” the authors wrote, more research on these infectious tumors “promises to illuminate important concepts underpinning cancer evolution.”

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