Some patients become convinced a stranger is someone they know, in disguise
People with Fregoli delusion believe that different people they encounter are actually one person, changing appearance to follow and watch them. First described in 1927, it's named after Leopoldo Fregoli, an Italian stage actor famous for rapidly switching costumes and identities mid-performance. Researchers think it involves a breakdown in how the brain separates familiar faces from unfamiliar ones.