Some animals catch cancer from others in their species – here’s why humans aren’t one of them

Scientists in Vermont have discovered a skin cancer spreading between catfish—only the fourth transmissible cancer ever found in nature. Catfish now join Tasmanian devils, dogs, and shellfish as the only animals known to carry cancers that pass between individuals. Unlike disease spread by viruses or parasites, these cancer cells simply break free from one body, survive the journey, and continue dividing in another host. The Conversation reports that transmission works differently by species: Tasmanian devils spread it through biting, dogs through mating, and shellfish through water. Transmissible cancers are extraordinarily rare because a healthy immune system destroys foreign cancer cells as invaders—the same reason organ transplants require immune-suppressing drugs. Genetic diversity also protects us: humans are so genetically different from one another that cancer cells from one person would almost certainly die before taking hold in another. The good news: there's no evidence of a naturally occurring, independently spreading cancer in humans, and studying these rare cancers in other animals may teach us more about how tumors evade the immune system.

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