The clams are dying. Since at least the 1970s, soft-shell clams (Mya arenaria) from Canada to Maryland have been plagued with a mysterious disease. This month, researchers announced that they’d finally identified the disease: clam leukemia.

Clam leukemia?!

Clams are simple creatures. They don’t have legs, or brains, or faces. But they do have hearts, and those hearts pump hemolymph (the clam version of blood) throughout their bodies. It’s in this hemolymph that the cancer has taken hold.

For years, scientists suspected that the clam-killing disease was caused by a virus. Then a team of researchers specializing in microbiology, cancer, and marine biology examined the clams’ DNA.

What they found inspired more questions than answers. The clams had cancer—and it was contagious.

Nearly all cancers are a one-shot deal, and can’t be passed from one animal to another. Until recently, there were only two known contagious cancers in the world. One is a dog STD called Canine Transmissible Venereal Tumor. The other is Devil Facial Tumor Disease, which is transmitted when one Tasmanian devil bites another one on the face.

But clams aren’t humping, and since they don’t have faces, they’re definitely not biting each other. How, then, is the cancer passed from one clam to the next? The researchers aren’t totally sure, but they think that the cancer cells may be released into the water. Clams are filter feeders, sucking up liters of water every hour. If a few clam-cer cells happened to be in the neighborhood, they could easily find themselves a new home.

Nobody knows how one clam’s leukemia could become contagious. So where did those free-floating cancer cells come from? To find out, the scientists sequenced cancer cells from clams all up and down the East Coast. Yet again, the answer was mind-boggling: the genes were all identical, which means that every clam’s cancer had all come from a single, original host. That’s one unfortunate clam—which got cancer more than 40 years ago. That clam’s cancer cells scooted off into the ocean, where they found another host, whose cancer cells eventually scooted off into the ocean … and now it’s 2015. The cancer has traveled hundreds of miles, and we’ve got a whole lot of sick clams.

And it may not just be clams. Mussels, oysters, and cockles are all afflicted with similarly mysterious illnesses. As long as people want to eat shellfish, these researchers will have jobs.

Thankfully, only clams can get clam-cer. Eating seafood and swimming in the ocean are still safe for humans. As safe as they’ve ever been, anyway.