If you've read mouses story the correlation between black Magic and infection is no longer an ambiguous idea, or it's ability to effect mortals, which is pretty much confirmed anyway when Rashid feels it's necessary to examine Harry personally, or list off incidents involving mortals. I think she's quite common, she's the devil in everyone's ear, everyone has a shadow for her to speak through, humans can just override it. Those who can't, quite clearly have their own shadow mirror inside reality to prevent Nemesis room to maneuver.
And thomas became addicted to poison soon after, he craved the love and wouldn't be well fed without it, that's not natural. Nor was Justine's original illness which Kravos likened to a drug addict, which was directly reasoned in book to reduce willpower and allow possession. She acted not only entirely different but in possession of mind reading abilities not natural to what she seems usually capable of. No, glossing over Justine's sickness is blinding oneself to deeper clues, I plain out don't believe medication does anything for her, and despite her tongue and cheek reply in WN that that's why she's normal now, it doesn't have anything to do with it, if she even actually takes medication besides Thomas, whom was literally her medication as per her description of his effect on her. A particularly large dose/bite seems far more likely and proven of an idea. Especially since we encountered another with similar issues in sarrisa... Same issues? Doubt Mab helped her with meds either.