Cancers can become parasitic life forms

A canine tumor turns out to be a parasitic form of a cancer cell from a dog that died 300-500 years ago. Now the hunt is on for additional forms of parasitic cancer:

The Loom : A Dead Dog Lives On (Inside New Dogs)

...One question the scientists do raise is how common such cancer parasites may be. Scientists have reported tumors that spread from transplanted organs, but these don't have a way to sustain their spread for centuries. Still, between mating, biting, and otherwise making contact, vertebrates provide plenty of opportunities for cancers to spread. And it is striking that our immune systems--and the immune systems of other jawed vertebrates--are equipped to battle so strongly against foreign tissues. What for? It's not as if Devonian-era sharks were giving each other liver transplants 400 million years ago. Perhaps, the scientists suggest, our ancestors had to fight against a different sort of tissue donation: cancer parasites.

In the dogs the parasite is relatively benign, in Tasmanian Devils a similar sort of parasite is lethal. This will be interesting ...