In this episode of Cold Cells, we open three case files on the most bizarre biological phenomenon most people have never heard of: transmissible cancer. We’re talking about a cancer in Tasmanian devils that spreads like a virus, a dog tumor that’s been alive for 11,000 years and has traveled every continent, and a clam leukemia that literally swims through the ocean to find its next victim.

This isn’t science fiction. All of it is real, all of it is happening right now, and the biology behind it — cell cycle breakdown, immune evasion, MHC class I, natural selection acting on a tumor like it’s its own organism — might be the key to understanding how our own cancers hide from us.

The case is open. The killer is still out there.