Japan’s Kyushu National Museum holds a copy of the Harikikigaki (Japanese:針聞書) — 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.
Well into the Modern Era (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.
Since ancient times, emotions and behavior are thought to be caused by insects (parasites) in Japan, such as worms.
When the Harikikigaki was written, it was considered to be a groundbreaking study of parasitology. This medical book features various parasites drawn in a colorful and unique way. Although dismissed by many today as youkai, it was generally taken very seriously. It is a remarkable find that consists on the subject of parasitology, unusual in Japan, and it is regarded as a very valuable book.