Mushi are insects, Jim, but not as we know them. As the Mushi-shi (a kind of blond, one-eyed ghost-buster) Ginko explains in episode one of the anime: "Put bluntly, it's like this. If my four fingers are animals and my thumb is a plant, man is at the tip of the middle finger, furthest away from the heart. The lower you are on the hand, the more primitive you become. If you trace them, the veins all converge around the wrist. Bacteria and microbes are halfway up the arm; at this point it's hard to distinguish between plants and animals. But there are things even lower than that. Up the arm, past the shoulder. Here are the mushi, the midorimono, the green things. Close to being mere essences of life, their shapes and essences are vague. Some can see them, some can't. A lot of so-called "ghosts" are actually mushi."
So far, Hisae and I have watched ten of the 26 anime episodes and the entire Mushishi feature film. The anime series is much better than the feature film (despite the presence of the lovely Aoi Yu in the movie). In the anime, each 25-minute episode is a model of clarity and atmosphere. The mushi-shi (a ghost buster, Shinto priest, shaman) arrives in a village afflicted by mushi-ghosts, uses his knowledge to cure the afflicted, explains a bit about the peculiar ghostly slime, and leaves. The live action film, on the other hand, tries to interweave all sorts of separate narrative strands, pasting over the cracks with CG animations, an orchestral score that tries to add spurious emotion to confusing, dark, tediously-miraculous scenes that should be under- rather than over-stated, and bashes the audience over the head with impact sounds that go "whoosh!" and "wallop!" These are, of course, tics endemic to epic films, and particularly Hollywood ones.
What's so nice about the anime series is that it's basically soothing and seductive. The episodic structure, the repetition with variations, gradually builds up our picture of a sort of alternative Shinto world in which animism becomes anime, and in which the hero's job is basically to make flying things a little less animated. Ginko does this by exorcisms of various sorts. Sometimes, even here, though, the explication gets clunky and arbitrary. As in Harry Potter and to some extent James Bond, the normal gravitational rules of narrative are suspended. Since we're in the thick of magic, anything is possible, though we know that our hero will never die. In this sort of narrative, the audience spends too much time learning all sorts of arcana which has simply been made up by the author (Yuki Urushibara, who published the original manga in Afternoon magazine from 1999 to 2008, according to Wikipedia).
If, in the land of magic, anything goes and the hero will inevitably save the day within 25 minutes, you basically want to spend your time enjoying the drawing, the characters, the atmospheres, the settings, and the weird inventiveness of the different sorts of inconvenience these primal flies can inflict (blindness and deafness are just the start). You don't want to clutter your brain with just-invented "rules" about this week's guest-star sub-bacteriological spectre.
Get on with it, ghost buster! Let's have another glimpse of that tatami room, that weird horned child, that sunlit coast, that snowy forest, while spooky Shinto gongs chime on the soundtrack!