... Not sure what issues Aurora or Catsyth had, not enough is known of either and they were already infected when we first meet them ...
As I wrote above: I think Aurora was tormented by the "cycles of suffering" between Summer/Winter, the constant warfare with humanity suffering not only Summer's cruel heat & stuff like Malaria, Ebola, and the other severe warm-location diseases, but also the rigors of Winter, the cold and the hunger and the hardship &c.
That (I surmise) was Nemesis' "temptation" to her, the way past her defenses... the chance to end the cycles.
It certainly seemed her prime motivation there at the end... so why not just as much Nemesis' "route in" ?
I don't think Cat Sith was nemfected when we met him; there was the scene at the Svartalf compound, where Sith didn't answer when Harry summoned him; that, I think was when he was being turned.
As to how/why: I think it was being Harry's "batsman," having to
obey a mortal that way... Think about it:
An obedient
cat?
An obedient...
malk??!?
The
Eldest Malk... obedient to a
mortal!!! (we need... like... a stabby-murder emoji here)
I think the rage and frustration, with no recourse or outlet, was Cat Sith's weakness for Nemesis to exploit. The chance to Kill Harry Dresden.
... That I think is a gray area, because when we first meet the Knights of the Cross and Denarians, on or two are killed and their coins released... Harry innocently goes to pick it up and if I remember correctly is stopped by Shiro, who warns Harry not to touch it, then carefully picks it up in a special hanky for lack of a better word. So I think you can be affected just by touching a coin even if you don't know what it is.
Ignorance isn't necessarily any defense, no. But you have to (at the least)
intentionally pick it up.
Harry was I think *particularly* vulnerable, there in the alleyway with Ursiel's Coin rolling loose: he is a magus, "one of the wise." He is supposed to take care, think before he acts.
Grabbing a random magical gewgaw, something associated with a monster so terrible that
all three KotC's had gathered to take it down? That is sheer arrogance... it is Pride, going before a Fall. Harry Dresden,
choosing to pick something up in his own Pride of Power, his "I can deal with this" arrogance. Oh, yes... Harry was
very much at risk, there.
Similarly the KotC's and other clued-in folk -- they have to take great care; because
choosing to be careless is enough of a Choice to leave them open to the Fallen within.
... So consider, when Maeve is shot, Nemesis is released from her body, of the beings present, who was most likely mentally and emotionally most vulnerable? Justine. So I agree that it isn't just a simple matter of being in the wrong place at the wrong time to get infected with Nemesis, it can also be true that being in the wrong place at the wrong time with vulnerabilities will get you infected... Both can be true at the same time.
Again, I go back to the Denarian Shadows. If someone dies with a Shadow inside them, that Shadow also dies; it isn't "released" to find a new host (at least, that's what Lash told Harry; but it also fits with our understanding of what a "shadow" is -- not a genuine entity but an "imprint" in the mind of one specific host -- when the host dies (really dead, not mostly dead) so does the imprint).
I (
strongly) suspect that the multiple instances of Nemesis follow that same principle -- if one of their hosts dies, so does that specific instance of Nemesis.
I honestly have no idea if there's a "Real" instance of Nemesis, a singular being who can be killed
really dead; or if it's just a bunch of parallel instances each of whom can act as the "full" entity, launching new Nemfections and acting with full power.