[Name] Hinamizawa Syndrome [Source] When They Cry/When The Cicadas Cry/Higurashi no Naku Koro ni [Symptoms] 
A mental disease similar to schizophrenia. It is often referred to as "The Curse of Oyashiro" by the townspeople who have no knowledge of the disease and instead relate it to a supposed curse on the town by its guardian god, Oyashiro. It causes paranoia-inducing hallucinations which progressively grow more threatening as the victim becomes more paranoid. In the case of the character Keiichi Maebara, it caused his friends' humored laughter to seem maniacal, and their tone of voice more threatening and sinister than it was. Eventually, he hallucinated a marker held by Mion Sonozaki to be a syringe, having heard that a murderer in the town was on drugs. This caused him to kill Mion, as well as Rena Ryuuguu, with a baseball bat. In most cases, the end result of the disease involves suicide, which Keiichi did by clawing his throat out. Rena also once attempted suicide from the syndrome by cutting her wrist with a knife. It is revealed later in the series that Hanyuu, the incarnation of Oyashiro, was responsible for some of the paranoia, mostly that one is being watched or followed. However, the disease was still mostly psychological.