Ah, good to make your acquaintance. Mushi are creatures unique to mine and Ginko's world, and are very largely spiritual in nature. Not everyone can see them, but they can have a physical effect on the world, especially in large numbers.

Ginko unfortunately attracts them as a natural part of who he is, and so we use incenses to help ward them away. Infestations still happen occasionally, and the cures tend to be a bit less...hmm, scientific in nature, I suppose.