Christie Wilcox in Quanta:
Aggressive cancers can spread so fiercely that they seem less like tissues gone wrong and more like invasive parasites looking to consume and then break free of their host. If a wild theory recently floated in Biology Direct is correct, something like that might indeed happen on rare occasions: Cancers that learn how to roam between hosts may gradually evolve into their own multicellular species. Researchers are now scrutinizing a peculiar group of marine parasites called myxosporeans to see whether they might be the first known example.
Even among microscopic parasites, myxosporeans are enigmatic. They were first discovered nearly two centuries ago, and more than 2,000 species are recognized today. Their complex life cycles make study particularly difficult: It wasn’t until the 1980s that scientists realized the ones found in fish were the same species as those found in worms, and not completely different classes of parasite. And while most parasites are content merely to snuggle into their animal host’s tissues, myxosporeans often take up residence inside a host’s own cells.
More here.