How did she say it works? (Rhetorical because you provided the quote). She didn't really say they were infected. Jim said they weren't. If it wasn't for Jim's statement, I'd assume they were infected. I'd still say we don't know. People on the forum have a tendency to ignore all of Harry's warnings about listening to the Sidhe. In context it's clear she's talking about Sells and Denton. Harry's repeatedly warned to not rely on context.
If I would get money for guessing right, my guess would be that Nemesis was behind Sells and Denton, but they weren't infected, just manipulated.
Just to add to your point, we now know that the Ladies are incapable of just giving a straight answer.
I'm in the camp that using dark magic corrupts a person with <insert term for the corruption>, which opens doors in the mind that otherwise remain shut. Nemesis/Beside can then whisper through those doors, twisting the individual without actually possessing them.
The Corruption would occur without Beside, but Beside can make it worse and guide it. I think that's what he did with Sells and Denton and the rest. He never possessed them fully, he just whispered and twisted using the corruption as a gateway.
But with Lea, Aurora, Maeve, Cat Sith, and Justine, there seems to have been some level of actual possession. At least for three of them. We're not 100% on Aurora or Maeve being possessed. They may have been corrupted and twisted by whispers, but not possessed. We only see Lea (on ice), Cat Sith (on boat), and Justine (on boat) be outright controlled.
So obviously, Harry should never go anywhere on a boat with anyone ever again. And also, maybe Justine is a scion or changeling of some sort after all, which allowed for the possession, whereas a full mortal human wouldn't due to free will laws. But I could also see where enough corruption and whispers could leave a human open to possession. After all, Before was in a human body (and a somewhat joking post-script WOJ is that it was Vito) and Behind was (briefly) in a human body.
