There is not a clear and agreed upon definition for any of these terms. They are all relative to what one is considering their opposite (i.e. healthy, uninfected, normal) all of which are also defined by their opposites, which is as circular as it sounds.
OCD is commonly called a mental illness and a neurological disorder, and both just mean “the person has a thing that interferes with their ability to live life the way they would like, and we think of that thing as being related to the mind/brain.”
In psychology there is a saying that you “treat the patient, not the symptom.” This refers to addressing the issues they identify as important and disruptive to them, rather than (as a silly example) fix their hearing if they hear voices but it doesn’t bother them.
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