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.”

In these rare cases, cancer becomes contagious when genetically identical cancer cells spread from one organism to another. Those cancer cells, like all cancer cells, contain genetic glitches that allow them to grow and reproduce uncontrollably, forming tumors. In many non-contagious cancer types, cells can break free of their origin tissue or tumor, spread through the body, then form tumors in new tissues and organs—a process called metastasis. But, in these infectious cancers, the cells not only migrate to new organs and tissues, but to whole new bodies.

Before the new report, researchers knew of only three cases of transmissible tumors: Canine transmissible venereal tumor, a sexually transmitted cancer in dogs and other canines; an infectious cancer in soft-shell clams; and the Tasmanian devil facial tumor disease (DFT1).

The latter causes deforming facial tumors in the devils, as well as tumors in the animals’ mouths, lymph systems, kidneys, and lungs. The cancer cells spread via bites from other devils, and the resulting tumors usually kill the animals within a few months of symptoms.

Since the cancer was first detected in 1996, the infectious tumors have spread across the island, and the devil population has declined sharply. In 2008, the devils were placed on the endangered species list.

The second type of infectious cancer, coined DFT2, looks almost exactly the same. The cancer cells spread through bites and cause similar facial tumors. But, the new cancer cells are genetically distinct. They don’t even seem to have similar DNA markers found in DFT1, dimming ideas that the second cancer is an offshoot or hybrid version of the first. Researchers report finding at least five devils with DFT2.

It’s possible that there’s something unique about Tasmanian devils that makes them particularly vulnerable to transmissible cancers, the authors speculate. The devils have low genetic diversity, common biting behaviors, and seem to be especially susceptible to cancers. But, the authors wrote, “the possibility that clonally transmissible cancers may arise more frequently in nature than previously considered warrants further investigation of the risk that such diseases could arise in humans.”

Proceedings of the National Academy of Sciences, 2015. DOI:10.1073/pnas.1519691113 (About DOIs).