Delusional parasitosis is a psychological disease that causes one to think that they are infested by parasites such as lice, mites, fleas, spiders and other organisms. Victims usually go from doctor to doctor finding no relief and rejecting any notion that the parasites are delusional. They believe in the infestation so much that self mutilation can occur from scratching, picking and other forms. Relief is only found when a victim accepts it as a mental illness and agrees to see a psychiatrist.
Source: UC Davis