Personally, I think that mortals have some "antibodies" to Nemsis in the form of Free Will (which is why Heaven protects that above all), but Nemesis corruption can still affect humans in the form of the "black magic corruption" wizards get by (e.g.) killing with magic.
Evidence (granted all circumstantial):
1) In Cold Days, Lily describes Harry's first three cases as the work of Nemesis. Granted, she may have been tricked by Maeve (how did she learn of those specific instances anyway?), but assuming we take her at face value, that means two things:
- Mortals can be influenced.
- Things we (and apparently Harry and Morgan) assumed were "merely" black magic taint were actually Nemesis. Specifically, we've seen Harry's feelings and impressions while using one of the belts. In his opinion, if he had killed while under it's influence he would have never been able to make it back.
2) The old "description of blackstaff healing reminds one of Mordite" comparison.
3) Of the seven Laws of Magic, 5 of them have to do with "black magic". If we assume that Nemesis==Black Magic, and further assume (yes, this is a stretch) that time magic is prohibited because of it's obvious use against Demonreach then all 7 Laws become "anti-nemesis" laws.
4) The flashback scene in Ghost Story can be read to imply that HHWB's goal in taunting Harry was to get him to go back and kill Justin (with Black Magic). If we assume that "starborn" is a two-edged sword (i.e. useful to Outsiders as well), HHWB might well have been trying to tempt Harry to Nemesis corruption.
5) Finally, there's my whole "(mortal) magic comes from Outsiders/Azatoth" WAG, where the similarities between "real" magic (Harry needs to believe that it works) and faith magic suggest common "divine" (but per Bob "different wavelength") origin. Combine this with the fact that only mortals can summon Outsiders and that their magic doesn't work well on them and you possible get an "Outsider God" as the possible source of magic.
Why Azathoth? Consider the following WoJ (Dragoncon 2010)
[On magic affecting technology] "It's something that changes over time.
It's a living growing force. It's not really aware, nothing like that, but it is something that changes along with the people who are using it."
**
IF** Jim is hinting at the source of (mortal) magic, then this is some sort of mindless divine power. To me this sounds a little bit like:
"the the ancient legends of Ultimate Chaos, at whose center sprawls the blind idiot god Azathoth, Lord of All Things"
.....
Ok, I started rambling. Nevermind, carry on.