I am amused by how many people think I'm saying mortals can't be Nemfected.
I'm only saying I think mortals get a choice in it. And that's the thing about choice: sometimes people choose badly.
Remember that the Deniarians can't subvert a mortal's free will. But they can persuade, tempt, seduce, bribe. Even when Harry touched the coin and had Lash in his brain working 24/7 to corrupt him, even then he had a choice. Not even a fallen angel can touch mortal free will: mortals always get to choose.
Even when Lasciel drove Harry to suicide, Uriel is careful to point out that suicide was still Harry's choice. Lasciel influenced mortal free will but did not subvert it — and even just that subversion was, itself, enough cause to get Uriel to come up off the bench and start directly taking a hand in matters.
If Outsiders can infect mortals and subvert their free will willy-nilly, where's Uriel? Where's Michael, Gabriel, and Raphael? The fact we haven't so far seen the Archangels get involved in any way against the Outsiders suggests strongly to me that the Outsiders are respecting free will.
They're just really, really good at tempting mortals into saying "yes"... Deniarian-level good at it.