I think Jim has dropped a few hints on this, but unless I am much mistaken I don't think this idea has been actually floated.
Any being becomes vulnerable to Nemesis infection via being too close to Mortals and the Mortal World.
But I think that only applies to beings that are not multiverse spanning - Archangels, Angels, perhaps the Mothers (guessed but unconfirmed). But the hint is in this WOJ:
Mortals, in their own possession of free will, have a tendancy to influence beings who don’t have it, in one way or another. I suppose it’s entirely possible, for example, that too much association with mortals are what changed Aurora, former Summer Lady, and gave her a determination to destroy the natural order in an effort to change its very nature–for the better, true, but it would never even have occurred to any of the other Queens, Mothers or Lady that such a thing might be, until it had already happened. It isn’t in their nature.
But perhaps I’ve said too much. I’ll shut up now.
So this first part I have highlighted pretty much gives that hint. We are pretty sure Nemesis is what corrupted her, but how it was able to get past her guard might be the fact she was spending too much time with mortals...like Elaine Mallory (make of that what you will...was it really chance she ran to the Summer Court...?)
And secondly, what is also really interesting is that Jim seems to be saying that the destruction of the natural order would be a
GOOD thing. Huh? Yet that second part I have put in bold seems to suggest that destroying the natural order might have been for the better. Could it be that the Faerie Courts are not as good for reality as we think? Could Nemesis serve a
purpose?
Now of course, that is somewhat unrelated to my opening but it is important. And I am aware that this doesn't really address how Mortals become vulnerable to Nemesis, unless of course by their very nature they are always vulnerable. By being able to Choose to do something, to shape themselves and their destiny via their Choices. I think so, and I think it is the same reason Immortals would otherwise not be vulnerable, but become so.
And it makes sense too! Lea spent too much time with Mortals, especially just before she received Morgana's Athame. Her immune system was down you might say (draws a nice parallel from Winter being Realities immune system, and the Outsiders being the virus). Maeve spent a lot of time with Lloyd Slate and Lily etc justs before Lea got to her. Cat Sith spent a whole bunch of time with Harry before HWWBf got to him.
Thoughts?