Delusional parasitosis is a form of psychosis in which sufferers hold a delusional belief they are infested with parasites.
It is usually diagnosed as a subtype of delusional disorder.
A related symptom involving a tactile hallucination of insects, snakes, or other vermin crawling over the skin is known as formication.
The sufferer typically reports parasites to exist under the skin, around or inside bodily openings, in the stomach or bowels and may include a belief that the parasites infest the sufferer's home, surroundings or clothing.
A person holding such a belief may approach doctors or dermatologists asking for treatment for the supposed infestation, and will often bring small particles, dust, skin flakes and other material for the doctor to inspect.