Hinamizawa Syndrome

The Hinamizawa Syndrome is a psychiatric disease from Higurashi no Naku Koro ni that is found in Hinamizawa and was researched by Hifumi Takano. His granddaughter, Miyo Takano, used the disease to infect the villagers with it and made it look like those infected by the disease were cursed by Oyashiro-sama as part of her plan to destroy Hinamizawa and surpass god himself (in fact, of course, she herself was infected by the disease since she had been to Hinamizawa several times). The syndrome is famous for its ability to make people infected by it go insane, causing anxiety and paranoia which can cause those infected to commit strange and violent acts, including suicide. It has been seen in an immunization shot in Oniakushi-hen, Meakashi-hen, and Tsumihoroboshi-hen. The symptoms of this disease are an amalgamation of real symptoms, such as clawing out one's own throat, which is a real symptom called formication.

Dr. Takano's Report on the Hinamizawa Syndrome
According to Matsuribayashi-hen, Dr. Takano had already discovered the existence of Hinamizawa Syndrome about 20 years prior to the story. He wrote a report regarding the town and its disease plaguing its residents and called it Hinamizawa Syndrome. His goal was to make the disease public, with an eye securing funds for its treatment and increasing the awareness of the general public. However, the higher officials didn't accept his report on the matter; to do so would mean accepting the blame of the Marco Polo Bridge Incident themselves. After Dr. Takano's death, his adopted granddaughter Miyo Takano decided to take over the research on Hinamizawa Syndrome, and received a great deal of funds from a mysterious woman named Nomura, representative of a underground organization called "Tokyo". It's implied through the some parts of the series that the disease may be related to some sort of parasitic organisms that live in the Hinamizawa residents.

Further Research in the Irie Institution
The research that takes place is based off of the collective data of the 2000 villagers and Dr. Takano's research in the military. Here they found that the Hinamizawa Syndrome's parasite abandons its host as soon as the host dies, leaving the researchers no way to identify the pathogen after the death of an infected person.

The syndrome has been shown to cause significant brain damage to the subject if they obtain L5 status. Hōjō Satoshi had an attack and achieved L5 status, but was hospitalized quickly afterwards. He was shown to be in a coma, or an induced coma for until his brain recovered enough with enough medication. Even after being treated for more than a year, Satoshi was in a state where if he woke up, he would continue to have delusions and hallucinations. Despite this, recovery is still possible if the treatment continues.