This tiny malevolent creature is believed to have caused human illness during the 16th century in Japan.
Gyochu is a deadly critter responsible for leprosy, acts as a messenger to the underworld. On the night of Koshin-no-hi (an important date occurring every 60 days on the Chinese calendar), Gyochu leaves the body to visit Enma-daio (Lord of the Underworld) and tell him your misdeeds.
Enma-daio is known to punish people for bad behavior by reducing their remaining time on earth.
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.