An 'Osei-chu' (mimicking roundworm) or 'Osei' (literally, mimicking or parroting the host's words) is a weird disease or a ghostly roundworm which causes the disease known in China and Japan, as described in collected essays written in the Edo period, such as "Shin Chomonju" (literally, a new collection of famous tales) and "Kanden jihitsu" (literally, fallow-field essays, continued; miscellaneous essays).
When a human is affected with this disease, he or she suffers a high fever for about 10 days, which is followed by getting a skin boil which gradually develops into the mouth shape. The mouth-shaped boil thus formed is named Osei-chu (mimicking worm), because it mimics the words of the host who suffers from this disease. Not only does it speak, but also it eats food. It demands food itself, and when it is rejected, it tortures the host with a high fever or hurls verbal abuse in a loud voice. According to the "Shin Chomonju," the disease is caused by a parasitic worm such as a roundworm living in the human body.
It is described as follows:
In 1703. A man living in Abura no-koji Alley got an Osei-chu boil on the surface of the stomach and continued suffering a high fever and verbal abuse. He tried out various drugs and theotherapy, but they did not work at all. A skilled doctor saw him and gave various drugs to the mouth of the boil. He selected some drugs from those that the mouth had rejected, and gave a mixture of them to the patient. The boil gradually became weak and 10 days later, a grotesque worm came out of the anus of the patient. It was an about 35 centimeter-long lizard-like roundworm, having an antenna in the head. When the worm came out and tried to run away, it was beaten up to be killed, and the patient recovered thereafter.
This anecdote is reportedly based on a true story of a real parasitic roundworm as described above. Abnormal hunger caused by a roundworm inside, and the body of a dead roundworm excreted from anus with vermifuge were allegedly attributed to a yokai (an imaginary weird creature). One theory has it that the story of Osei-chu has its origin in China and that the Japanese Osei-chu was created based on translation and dramatization of the Chinese original.
In addition, in "Kanden zuihitsu" (literally, fallow-field essays; miscellaneous essays), the following story was described. In 1738, a sideshow manager doing business in the Abura no-koji Alley heard about a woman who seemed to be caught by the Osei-chu and visited her to recruit her into their show. When the manager saw the woman, he noticed that surely she suffered from the Osei-chu because they heard the voice from her stomach. Her husband disclosed that when they went to worship at a temple, people around her were suspicious of the voice coming from her stomach, which was so embarrassing. The husband continued that she would therefore not be caught dead being in the show. In this way, the manager's expectations were disappointed.