This tiny malevolent creature was believed to have caused human illness during 16th century in Japan.
Koshi-no-mushi is described as a small fly with multiple sections. It flies into a host's body and makes its way to the lower back area, where it causes diarrhea, extreme sweating and chest pains.
The herbs mokko and kanzo (licorice root) are an effective treatment.
In the Harikigaki
Japan’s Kyushu National Museum holds a copy of the Harikikigaki — a 16th century medical text of unknown authorship which claimed that diseases were caused by tiny bugs that crawled into the body. The Harikikigaki advises using acupuncture and herbs to deal with the bugs.
Until the late 19th century, Japanese people believed illness was spread by evil kami called yakubyogami. At first these gods were thought to take human form, but later, influenced by thinking in texts from China, some people came to think of them as little creatures as small enough to enter the body. The Harikikigaki, written in 1568, is mostly about acupuncture, however, this rare text includes 63 color depictions of the various mushi (germs) believed to cause diseases.