Paranoia can be a symptom of several illnesses including schizophrenia, brief psychosis, paranoid personality, psychotic depression, mania with psychotic features, or substance abuse, chronic or momentary. It can range in intensity from a character style to a severe impairment.
One form of paranoia that is particularly difficult to diagnose and treat is Delusional Disorder of the Persecutory Type. In DDPT, the sufferer is gripped by a delusion (a fixed false belief) that involves a singular situation or person—a “circumscribed” delusion. A wife “knows” that her husband is cheating on her with the neighbor, a person is convinced a co-worker is snooping in his desk, a manager is clear that employees are plotting to get her fired, an adult child is immovable in his belief that his father’s new wife cut him out of the will. The fixed false belief plays out around this one notion or person, while in other ways the afflicted may function just fine.