I'm not sure how I feel about using labels like "parasite", I've seen and known things that put a lot of such typical new age labeling of harmful spirits in a different light (foolish, harmful, and possibly deeply immoral)...
Anyway, I believe "is my magic not working due to negative spiritual influence"
is a valid question, and simply considering the answer to be true can lead one to solutions, regardless of whether it was one's own fears, or someone's curse, or an "evil" spirit sucking one's blood, or in the case of all of these being the same thing.
I'm very much a skeptic, and a proponent of building good "discernment", but in many cases it can develop into a very post-Enlightenment western positivist slicing-and-dicing mentality. "Make sure it's not X before thinking of Y" implies that Y can't be also X, or X can't "cause" Y.
Ie. if someone is cursed, only on the rarest occasion will they burst into fire in an open street, and be dragged to hell by screaming banshees. Maybe instead their emotional weaknesses become more emphasized, and their vices become harder to control. The post-Enlightenment mage might say "you're not cursed, you're just a neurotic asshole"... but what if being a neurotic asshole
is the curse?
