The problem here being that people who become ill from a real parasite infection are the most likely people to acquire a delusion that exaggerates and extends t

The problem here being that people who become ill from a real parasite infection are the most likely people to acquire a delusion that exaggerates and extends their symptomatic distress; The feeling of 'formication' is both caused by insects crawling on the skin, and caused psychogenically by the self-reinforcing expectation that insects are crawling on the skin, an expectation that is entirely rational in anyone who has been subject to an infestation. Detecting parasite infection is non-trivial and error-prone. A family member has enduring beliefs and anxiety about head lice, which drove them to psychosis for a period of six months, causing all sorts of little false beliefs and reactive rituals that persist several years later as a sort of PTSD. The fact that they quite likely had lice at one point that triggered this whole thing, is no longer provable, but if examined without this case history, we would dismiss it as some kind of waste of physician time. It is very hard to convincingly reassure a patient that they 'no longer' have an infestation that they once had, but are still feeling the symptoms of.