Though cthulu and the necronomicon of the mad arab Abdul are more often associated with any tale from Lovecraft's stable, there are a couple of astounding nuggets to be found in his short stories that have an undercurrent of the morbid and macabre.
One such story is called "The Shunned House" - a typical theme of a supposedly haunted house replete with a history of unexplained deaths, now dilapidated but yet draws our protagonists into a lengthy and costly investigation into its macabre past. After reading quite a few Lovecraft stories, I see that most movies have popularized his dark psychological terror theme. So a movie buff may not find a twist to the tale so to speak but has found a treasure trove of good old story telling, not to mention a very dark evil brooding terror that waits in the dark recesses of your mind, biding its time to unleash its numbing horror on your psyche.
This is THE best attempt at a rational explanation for the existence of parasites at a spiritual level. One can be anal and argue some points from a technical and scientific standpoint but I urge you to gloss over them and appreciate the entireity of the argument - its language and the poetic way in which Lovecraft crafts his terror.
"Scientific study and reflection has taught us that the known universe of three dimensions embraces the merest fraction of the whole cosmos of substance and energy. In this case an overwhelming preponderence of evidence from numerous authentic sources points to the tenacious existence of certain forces of great power, and so far as the human point of view is concerned, of great malignancy. To say that we believed in vampires or werewolves would be a carelessly inclusive statement. Rather it must be said that we were not prepared to deny the possibility of certain unfamiliar and unclassified modifications of vital force and attenuated matter; existing very infrequently in three dimensional space because of its more intimate connection with other spatial units, yet close enough to the boundary of our own to furnish us occasional manifestations which we, for lack of a proper vantage point, may never hope to understand.
Such a thing was surely not a physical or biochemical impossibility in the light of a newer science which includes the theory of relativity and intra-atomic action. One might easily imagine an alien nucleus of substance or energy, formless or otherwise, kept alive by imperceptible or immaterial subtractions from the life force or bodily tissue and fluids of other more palpably living things into which it penetrates and wth whose fabric it sometimes completely merges itself. It might be actively hostile or may be dictated merely by blind motives of self preservation. In any case such a monster must be in our scheme of things, an anamoly, an intruder whose extirpation forms the primary duty of every man not an enemy of the world's life, health and sanity.
It might be pure energy - a form ethereal and outside the realm of substance - or it might be partly material; some unknown and equivocal mass of plasticity, capable of changing at will to nebulous approximations of solid, liquid, gaseous or tenuously unparticled states."