Q,
I've been thinking about this and I'm having a hard time buying the effects of black magic being a form of Nemfection.
I get it that using black magic can drive someone ... off center (like the Korean kid in PG), but that could be something similar to going crazy from a syphilis infection.
We know Nemesis is an outsider with an intelligence and a purpose. We know that Nemesis can vary the amount of control it exerts over one it has infected. We don't know if it takes a volunteer to be an Aurora or Maeve type of infection, or if it's really more like a Cat-Sith infection, and is only masked.
We saw the Leansidhe fighting it, but we don't know how much influence it had before she realized her error and asked for help.
I'm thinking Nemesis is an infection and has a scale of control from Aurora (barely whispering in her hear), through Maeve (probably talking like Lash did), to total full on possession like Cat-sith.
I just don't see it as a general virus like Syphilis.