That's sort of the point... Nobody knows. The Legendaries don't even know. It's probably some sort of microcopic organism that multiplies itself or otherwise infects living things, but part of the point of the RP is that the characters are fighting something you don't know anything about.
So, wait... You're randomly asking for me to post in your thread without knowing whether or not I'm interested? Considering I happen to lurk about the Writer's Lounge often and have seen your thread before, I would have posted if I really wanted to. x3; But, I guess I'll do it anyways... even though I really don't like blatant thread-advertising. D: