The world parasite is a global virus present in 99% of the human race, it is also hypothesized by the UN and Takano Hifumi to be the cause of all cultural and religious violence in the world.
Discovered by the UN decades before Dr. Takano's research into the Hinamizawa Syndrome, the parasite was found to be inside 99% of the human race, but the information was deemed too dangerous and the findings were classified and stowed away. It was later brought forward by Nomura and the Alphabet Council to the Japanese government during the quarantine of Hinamizawa and it's surrounding areas in Higurashi no Naku Koro ni Kaku. Nomura then states that the Hinamizawa Syndrome is only one parasite strain of hundreds present throughout the entire world, and that we are all infected. Hanyū further calls those who succumb to L5 her brethren (In reference to her tribe), hinting that the parasite is only the physical manifestation of a demon spirit, or somehow related to the witches from Umineko.