The Fregoli sydrome is a “monothematic delusional” disorder in which a person holds the belief that different individuals are in fact a single individual who morphs their appearance simply to deceive them or those around them. It is often characterized by paranoia, with the affected person believing themselves followed by the deceiver.
According to Kaplan & Sadock’s synopsis of psychiatry the disorder is not only rare, but possibly related to schizophrenia, dementia and epilepsy. One of the only believed direct causes is brain lesions, although there have been a few cases directly appearing after childbirth.
The first case of Fregoli Delusion was noted in a 1927 paper entitled: Syndrome d’illusion de Frégoli et schizophrénie. In which, they described the plight of a young woman of 27 years who believed that two actresses were stalking her. According to P. Courbon and G. Fail, she believed that:
She is the victim of enemies, of whom the main culprits are the actresses Robine and Sarah Bernhardt, whom she often went to see in the theatre. For year they have pursued her closely, taking the form of people she know or meets, taking over her thoughts, preventing her from doing this or that…
Since then very few cases have been discovered, but they are easily treated with medication and therapy.