How many "vectors" do we know of for sure?
There's the Athame -- Bianca gave it to Lea, and it infected Lea, who infected Maeve (we don't know if Maeve was infected by Lea herself, or the Athame, or Lea-wielding-Athame; but it was a single "vector" of infection).
That's the only one we can really pin down as 100% known, I think.
The Athame then went to Mab, who (we think) didn't get Nemfected. We don't know if she purged the Athame, or what; eventually, we stopped seeing it.
We don't know where the Athame came from, or who/what else it might have infected before Bianca gave it to Lea. Presumably it's now "gone," no longer a vector of Nemfection... unless Mab herself is.
Cat Sith was infected. Personally, I suspect a "slow burn" infection from the Athame/Lea/Maeve vector before Mab, but we don't know -- it could have been a new vector. Or maybe Mab thinks she has it contained, but she doesn't, and somehow it got Sith. I've seen some folks suggest Sith's Nemfection came in the middle of the novel, offscreen (I tend to suspect not, that Sith was infected from the very first time he appears in Cold Days.), but again: we don't really know, there's been no unambiguous evidence.
There was Aurora's Nemfection. Maybe the Athame was in Aurora's hands, before Bianca got it? Can't rule that out, I suppose... though I'm in favor of Trojan Elaine as Aurora's vector, personally.
Any other fae?
There was Vittorio's "possession" (but I think it was one of the HWWB brothers, not Nemesis).
Any other supernatural's?
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Then there's the open question of mortals being Nemfect'able. I'm firmly settled in the camp of "maybe," but for purposes of this discussion, I'm arguing from the "yes they can" POV, because otherwise every anser is "no," and that's just boring (which in itself is IMHO a good argument that Jim will let mortals be Nemfected).
Was there a vector that got Sells, and/or Kravos? Etc. Or were they "just" warlocks with "mere" black magic? I've seen it suggested that ALL black-magic is Outsider-y. I s'pose that's possible.
I've theorized that -- since what Nemfection seems to do with/to Faeries is give them something like "Free Will" (the ability to act against their essential nature) -- perhaps mind-magic is specifically a Nemfection-vector (at least, potentially so).
Following up this theory, maybe each Law of Magic has a particular Outsider agent which gets a shot at wizards violating that Law. Maybe each Outsider is tasked with undermining reality via that channel; maybe each Law is specifically established to block that particular Outsider's access to Creation.
Anyone else want to weigh in -- or WAG in -- on these questions?