Hinamizawa Syndrome
Hinamizawa Syndrome (雛見沢症候群, Hinamizawa Shōkōgun) is a psychiatric disease found exclusively in citizens of Hinamizawa. The most common symptoms include anxiety and paranoia, which can force the afflicted to commit strange and violent acts. Other symptoms include an extreme irritation of the lymph nodes and formication.
Causes
Queen carrier theory
Hifumi Takano identified an antibody of Hinamizawa Syndrome in members of the Furude family. He used this to designate the family as the as the "queen carriers" of the parasite, with the status of "queen carrier" falling upon the latest born female. He theorized that 48 hours after the death of the queen carrier, the parasites afflicting the villagers will aggravate the symptoms of Hinamizawa Syndrome, forcing everyone in Hinamizawa into L5 status. To counteract this, Takano created Emergency Manual #34, a contingency plan to wipe out the entire village should a queen carrier die.
However, in both Watanagashi-hen and Meakashi-hen, the queen carrier, Rika Furude, is dead for over 48 hours with no effect. This means that the queen carrier theory is either partially or entirely incorrect.