Some have questioned whether there was contagion going back that far for Maeve, it just hadn't gotten to critical mass yet. This seems to be contradicted by text of Mab suggesting athame and Lea as directly responsible for nemfection though. Who knows though with absolute certainty when it comes to Nemesis.
Mab herself may be infected, it just hasn't reached critical mass whereby it can subvert the mantle's imperatives. Think of it as a virus load. You can have a virus with no symptoms. Only when the virus load reaches a certain amount, can that virus express itself in a set of symptoms. And with this particular infection (nemfection), their exists an ability to choose to overcome virus simply by wanting to overcome virus and access to treatment.
So, Lea could still be infected with Nemesis, it's just that her viral load is extremely low and now she has some immunity/resistance to the infection now.