When did Maeve handle it?
"A few years back, you got angry. So angry that when you spoke it made people bleed from the ears. That was why. Because you figured out the the adversary had taken Maeve. And it hurt. To know that the adversary had gotten to her."
"It was the knife." Mab said.
"Knife?"
"Morgana's athame," Mab said in a neutral tone-but her eyes were far away.
"The one given to her by the Red Court at Bianca's masquerade. That was how the Leanansidhe was tainted-and your godmother spread it to Maeve before I could set it right."
There is no reason to think Maeve physically handled the knife. It seems more likely to me that Lea found a way to directly infect Maeve.
There is another related idea that I think needs to be challenged. Because Mab wore the Athame on her belt, it is easy to conclude that Mab has been exposed to Nemesis. I think that is incorrect and I will say why further below. First, I suppose an alternative to this thought is that Mab found some way to shield herself from the infection, but that seems unlikely to me because it's too simple and too easy. If Mab could protect herself than Nemesis is no longer so scary a weapon.
A more subtle alternative is that the Athame's nemesis infection was a one shot item. Mab could wear the Athame because it had lost its power to do harm to the one who uses it. I think that is also an unlikely explanation; however, if you reread the previous sentence I just wrote, the clue to how I think the Athame works is found at the end of that sentence. I'll just wait while you reread that.....
I think Lea didn't become nemfected until she used the Athame to summon everyone in Winter who owed her a favor to capture Harry, near the end of Grave Peril. If simply touching the item passed the infection, it would have been all too easy for Bianca to infect herself, assuming she was just a patsy and not the mastermind of the operation. I can't describe it in exact terms, but something about using the Athame's magic passes the poison to its user. So it makes sense for Mab to permanently keep hold of the item. It can't harm her as long as she doesn't use it and it can't harm anyone else either.