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 occurances which may be related to the "Mushi".
Hannah's comments: Fun to see a sort of a ghost animé, ghosts which are shaped mostly as Japanese characters, but are ghosts in a real form and which can change the real life. Those mushis, can be evil or quite harmless. Not so common to watch such an animé like this. In general I liked the Mushi-Shi very much, but there are some things I disliked. I didn´t like that people and the environment nearly looked the same through the whole series, even if Ginko (the man with silver hair) travelled all over Japan, the villages, mountains and persons looked over all the same; it was only the story itself and the mushis which were clearly different. Mostly good animated and soft drawn, especially the mountains and the environment. The persons were drawn "so-so". The music was more than ok, suited good to most of the episodes. Sometimes hard to follow, because of the strange mushis behaviour and the purpose of their lifes. Definitely a worth watching animé, which also can be watched even more than just once. I think you can understand the Mushi-Shi even more.