In Japan, ten years ago, Mushi appeared. No one knows where they came from, nor what exactly they are, but they look like bugs/insects, and latch on to a person and feed on that person's dream. Some people say it's a reward, others say it's a punishment, but people that get mushi, gain ability through their bug.
To fight this threat and to keep the further information of Mushi a secret, the japanese government formed a special environment protection agency to study the phenomenon.
As for the people that get Mushi, when the Mushi finishes eating a person's dream, the host dies and the bug becomes a grown mushi, gaining unparalleled power and durability.