I think that if Peabody were Infected, he would have spread Nemesis to the Senior Council ASAP. He was using mind-affecting ink instead of transmitting Nemesis itself, which would be the logical way to do things, in my opinion.
Though that may be explainable by the process of Nemesis Infection itself. Jim hasn't revealed the precise method of transmission. If it requires overpowering someone and holding them down for an hour or something, then that's going to be difficult for someone like Peabody.
What we know about that:
1. It should be possible to spread Nemesis via an inorganic object. The object itself may or may not need to be sufficiently powerful on its own. The Athame could have been used for a couple of reasons that I can see: either it's necessary for the object to have a certain level of power (Athame, the Swords, relics like the Holy Grail or some other kind of artifacts), or it simply had to be powerful enough to tempt Lea into trading for it.
2. It's possible to Infect someone quickly, like they did with Cat Sith. He was off-page for about, what, five hours? Ten? Cold Days all happens within a 30ish-hour period (Harry leaves the party for Chicago sometime after midnight but before dawn, and the final confrontation on Demonreach is before Dawn on the following day. 2-3am-->1-5am, in my opinion). But the point is that, between the time Harry first talks to Lily and Nemesis is revealed and the Wild Hunt v. Outsiders Cage Match, Cat Sith is captured and Infected.
3. Those who are Infected can be made aware of their own Infection. This means that Nemesis works best when it lays low for a long time, slowly altering the Vector until they're behaving in the way Nemesis wants. When Cat Sith was confronted with his out-of-character behavior, Nemesis was forced to take direct control. I take that to indicate that he was a rushed job, because we haven't seen that behavior from other confirmed Infected (namely, Maeve).
4. There is likely some kind of cost for Nemesis to spread to too many things. This is speculation, but I think that, if Infection was a free action for them, then they wouldn't have stopped with Lea and Maeve. I think that most of Winter would've been Infected. Maeve was Infected for years, and she has the time, power, and clout necessary to go around and Infect countless other Fae, but didn't. Either that means that there is a cost to spreading itself to too many different entities, or doing so would have had undesirable results—like maybe the Infection would've been noticed and stopped sooner.
It's also noteworthy that Lily wasn't Infected, despite the fact that Maeve spent lots of time with her, and doing so would've been easy enough. Instead, Nemesis opted to manipulate her. Why not Infect the new Summer Lady? Why is manipulation a better option? Surely it would've been advantageous to have both Ladies Infected and influence both sides of the Fae, right?
I therefore infer that there is some kind of cost associated with Infection. Either Nemesis can maintain a limited number of active Infections, or there is some other kind of cost I can't conceive at the moment.
5. No mortals have been confirmed to be Infected. Some insist that mortals can't be Infected at all, but I don't see why that would be the case, and haven't seen any evidence beyond "No confirmation of Infected Mortals = Can't Be Done." Someone please correct me if I'm wrong.
6. The most powerful entity that is confirmed to be Infected was, at one point, Lea (in her own words, she's second only to Mab in Winter). Mab tortured her (or, at the very least, put her through "exquisite pain") to "cure" her of the Nemesis Infection. That means that, even if it can't be fully cured, the symptoms can be treated if the host wants to change. Lea is shown to have the "mad glint" in her eye once, briefly, after her treatment by Mab. It is possible that Nemesis remains within Lea, but is currently held at bay by something (maybe even simple negative stimulus conditioning, like that "How to Stop Being a Freemason" Monty Python sketch). It's also possible that Nemesis is gone, but the effect on Lea still lingers, and she must make the occasional conscious effort to suppress it.
That is all that I can recall about Nemesis.
On topic, I think that the likeliest mortals to be Infected in the future are:
1. Senior Council members. Any of them, really.
2. Kumori, if she isn't already. Cowl, too.
3. Fix. I think he'd be a good target.
4. I don't think it will happen, but Infecting Murphy would break Harry just as badly as her death would. In fact, it might be worse, because he'd be crazy enough to hope to fix it.
Those who may already be Infected:
1. Elaine.
2. I hate to say it, but Justine.
3. Christos.
I say Justine because of the shape of her character arc. She's gone through some radical changes throughout the series. Like, really, really big ones. In fact, when she reveals that she's regained her faculties to Harry (in White Night? I can't recall), she simply states that there's no time to explain how she's recovered, and it's never revisited. And now she's having Thomas's baby, which gives her leverage over Thomas, the White Court, and Harry. I think it's possible that, during her period of disability, she was Infected, and Nemesis managed to not only help her recover faster, but increase her influence in the White Court. Can you think of another mortal who worked so closely with anyone in the upper echelons of the White Court? Certainly not for as long as she has. Lara defends her with her life, and not just because Justine is important to Thomas.
That's my WAG about it, anyway.
