Nemfection is pretty clearly a code-insertion attack, with individual filespeople getting corrupted & propagating the infection when they run ...
Yes, that is pretty much confirmed by the fact that Rashid "x-rays" every soldier with his eye as they return through the Gates after a skirmish. Either he misses one now and again, which I doubt, or Nemesis gets in our world another way, which it does. It relies on mortals who are willing to summon it knowingly or unknowingly with they play with the dark arts.