Mushi Master Ginko travels from place to place investigating occurrences that could be tied to supernatural creatures called Mushi.

"Mushi": the most basic forms of life in the world. They exist without any goals or purposes aside from simply "being." They are beyond the shackles of the words "good" and "evil." Mushi can exist in countless forms and are capable of mimicking things from the natural world such as plants, diseases, and even phenomena like rainbows. This is, however, just a vague definition of these entities that inhabit the vibrant world of Mushishi, as to even call them a form of life would be an oversimplification. Detailed information on Mushi is scarce because the majority of humans are unaware of their existence. So what are Mushi and why do they exist? This is the question that a "Mushishi," Ginko, ponders constantly. Mushishi are those who research Mushi in hopes of understanding their place in the world's hierarchy of life.—MAL Rewrite

They are neither plants nor animals. They differ from other forms of life such as the micro-organisms and the fungi. Instead they resemble the primeval body of life and are generally known as "Mushi". Their existence and appearance are unknown to many and only a limited number of humans are aware of them. Ginko is a "Mushi-shi" who travels around to investigate and find out more about the "Mushi". In the process, he also lends a helping hand to people who face problems with supernatural occurrences which may be related to the "Mushi".—Anime News Network