I don't see her being a parasite infected from outside, with an established means for how (the Denarian coin) necessarily being of any definitive relevance to the case of White Court "demons"; we have no evidence for any ways of getting them into otherwise uninfected humans.
I don't think that's an accident, any more than I think it's incidental that the White Court does its business in Etruscan, or that the titles of the BAT are the favorite White Court curse words.
My
guess is that the first White Court vampire was a human being or small group of human beings, who (consensually or not) was merged with one of the demon-things, probably in Etruria in pre-Roman/early Roman times, and the modern White Court are descended from him, her, or possibly them, if it was a small group.
Was Lord Raith that first one? Theoretically possible, but I rather doubt it. But I doubt he's many generations removed.
Which suggests a linkage to the Black Court more than the Red, oddly. The Red Court appear to be
physical parasites, they use humans as hosts for their offspring and the offspring copies some of the personality and at least some of the memories of the deceased host. For a mundane world metaphor, they are like the insects that plant their offspring inside another creature, who devour it from within. The emerged Red Vampire is in no sense the same entity that as the former host human, who is gone.
But the Black Court vampires appear to be the same human entity, dead and reanimated, true, full-deal
vampires. In that sense, they are more like the White Vampires than the Red. The White parasites seem to leave the host alive, and need the host alive, and reproduce when the host reproduces. It looks suspiciously like a link to the Outside is involved.
The Black Court kill the human, but reanimate him/her with some even deeper link to Outside. It's not obvious that the Red Court had an Outsider link at all.
Now I can't prove this, but it's my working hypothesis. I also note that Harry muses at one point that the White Council dates back to that period (in earlier form, no doubt). Is there a link? Insufficient data.