Cancer is typically a one-host dead end, but a small number of cancers have evolved to do the unthinkable: spread between individuals like infectious diseases. Researchers have now confirmed transmissible cancers in four animal groups — dogs, Tasmanian devils, clams, and, most recently, brown bullhead catfish — and scientists believe more may be hiding in plain sight.

Each case reveals a different survival strategy. Dog and devil cancers suppress MHC-I proteins — the molecular "ID badges" immune systems use to flag foreign cells — while clam cancers thrive partly because bivalves lack MHC-I altogether, allowing tumor cells to float freely through seawater. In catfish, a newly reported melanoma (published in Nature, July 2026) appears to spread during spawning aggregations, and may have been circulating undetected since Henry David Thoreau described similar lesions in the 1850s.

Key Takeaways:

• Transmissible cancers evade host immunity via MHC-I suppression or absence — a mechanism with potential implications for understanding immune evasion in human cancers
• Tasmanian devils are evolving a broken variant of the Saha-UA gene that helps reject tumor cells, offering a natural model of cancer resistance
• Softshell clams on the U.S. East Coast have developed mysterious defenses against their leukemia-like cancer, and the same cancer lineage has now jumped to Puget Sound on the West Coast

Why it matters: These transmissible cancers are more than biological curiosities — they're natural experiments in immune evasion, cancer evolution, and speciation. Understanding how they spread and how hosts resist them could illuminate new strategies for fighting cancer in humans.