I think that there also maybe some ritual, or choice of rituals, that takes place to infect someone.   I think one of the possible rituals is involved in sex magic and that is why Maeve wanted to seduce Hunt-Master Harry so bad throughout the whole series.  I think Lea using the Dagger infected her, and she infected Maeve.  That means there are two ways of being infected.  Contact with an infected magic item. Some sort of contact with an infected person.  However it seems it requires more than just casual contact.
I also think that Nemesis, a thinking being, is somewhat selective in its process of who it infects and probably doesn't just go after every small fish in the sea.  The spectrum is that there is all kinds of carriers that only are triggered if they need to be.  Kind of like a magical Manchurian Candidate type thing.

I am trying to contact Jim or whomever handles his customer service issues.  I recently downloaded "Death Masks" audio version from audible.com and it was missing chapter 33 the final chapter.  I am huge fan and have read all his box multiple time so when the audio book ended with Dresden being pulled out the river I was extremely pissed since the events of the next chapter hugely shapes the next several books until Dresden gets his headache from Cowl.  Audible.com was professional and gave me a refund, but I thought I would drop you all a line and but a buzz in your ear.  I am doing this because the impression I have gotten about Jim is that he would be upset to have his work sold incomplete and maybe you all can work with penguin audio and audible.com to fix the problem.

Thank you,

A. J. Nicholson
Knoxville, TN