They are neither plants nor animals. They differ from other forms of life such as micro-organisms and fungi. Instead they resemble the primeval body of life and are generally known as Mushi. Their existence and appearance are mostly unknown and only a limited number of humans are aware of them. Ginko is one of these select few, a Mushi-shi, who travels around to investigate the Mushi. In the process, he also lends a helping hand to people who face problems with supernatural occurrences that may be related to the Mushi.

Source: AnimeNfo

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Sequel: Mushishi Tokubetsu Hen: Hihamu Kage