... I wonder how Aurora got Nfected - who would’ve been able to get to her? I understand Maeve was Nfected through Lea, who in turn was Nfected by the athame given to her by Cowl & co. I wonder who the Summer representative at Bianca’s ball was...
Well,
damn. There's an obvious answer, here; I don't like it much, but it looks pretty damning...
Nemfection is presumed (or known? Do we have WOJ or unambiguous proof in the stories?) to be an Outsider thing.
Du Morne was into Outsiders. He summoned a
freakin' Walker to go kill an errant apprentice (which normally would be like a thermonuclear flyswatter (except Starborn)).
Du Morne spent a day messing with Elaine's mind (as prelude to taking Harry's mind too); a side-helping of Nemfection seems not only possible, but likely. As soon as she got loose(ish) from the Enthrallment, she went straight to the Summer Court, asking the Summer Lady for shelter. There's your vector.
Let's think for a moment. Most of Harry's actual training came from DuMorne; I presume that includes much of what he learned about the Summer&Winter Courts; Elaine likely had the same training. And Harry's
gut reaction (deeply trained) to the Fae is that they are dangerous, deceptive, & untrustworthy. Elaine was supposedly frightened and confused as she fled from the Harry-vs-Justin smackdown; not the time I'd expect her to turn to any Queen of either court -- even the most approachable (the Summer Lady) for succor!
Nevertheless, that's what she did... and shortly thereafter, Aurora was shown to be Nemfected.
Outsider-summoning Justin DuMorne, creating a tool out of Elaine Mallory, Nemfecting the Summer Lady. It doesn't even call for Justin to be a knowing part of this; Nemfection can apparently be quite subtle, so it may have happened without him noticing.