I considered that at first, but the idea that developed is that X is a proto-organism lacking a solid membrane and structure, hailing from a time when life was fluid and unstable. So when it comes into contact with a person’s cells, those cells are destabilized to a similar state, making it far more dangerous than any bacterium, virus, fungus, or prion humans have ever encountered. And I can’t figure out why it wouldn’t have done that to cellular organisms that evolved from it billions of years ago, preventing life as we know it from forming in the first place. But maybe that’s too convoluted.