Nemesis – Invidia. (latin) for envy, one of the seaven deadly sins.
Wouldn't that be interesting if Nemesis was in fact how Nemesis was transferred? When a being becomes envious?
It would make a lot of sense when we consider Lea.
Lea has long been Mab's greatest rival, and most powerful servant, within the Winter Court. Mab has allowed her to gain significant Power whilst also incurring commensurate debt. Lea would both hate and envy Mab's power and position. She has to be one of the most ambitious of the Fae, as she would not have grown her power so much otherwise. In Dead Beat she gains Morgana's Athame, a weapon tainted with Nemesis, but still grant her enough power that she unbalanced the Winter Court and was a real threat to Mab's authority. But the fact that she openly challenged Mab at all is perhaps what is most strange, as even Lea wouldn't risk unbalancing the Winter Court normally. The Fae seem to like subterfuge, sex, and brutal violence when it comes to internal matter (although externally this also seems to be the case...).
Aurora too perhaps was envious, not of Titania's power I think, but of Mortal freedom. She was tired of being immortal and the endless cycle of death and rebirth.
Maeve of course was envious both of Sarissa's place in Mab's eyes, but also to a lesser extent, Mab's power and position. It is made pretty clear on Cold Days it is the fact that Sarissa is free-er and seemingly more loved by Mab than Maeve, that is the root of her anger and sadness.
Cat Sith...well who knows. Probably envious of the power of mightier beings in the Court, and deeply resentful of being beholden to Dresden. Perhaps envious of Dresden's higher status.
I am leaving out the mortals who Lily accused of being infected by Nemesis, as I don't want the discussion to be derailled and I don't think I will convince anyone that there is a good chance that the mortal's are infected. Let's just stick to the "known" infected candidates.
Also, whilst I am NOT saying that Nemesis IS one of the seven deadly sins, if that were the case then it would makes sense that there are other sins (and perhaps corresponding virtues) that are sentient beings. Food for thought.