Cause
Musophobia is caused by the unconscious as a protective mechanism. This mechanism was probably created as some point in the persons past when they had a traumatic experience with a mouse or rat. Examples of this could be having their house or room invaded by them, finding them eating the person's food, being surprised when they jump from a trash can, smelling them, or getting sick from them (The Black Plague in Europe was carried by rats).
This fear could be triggered by the presence of a mouse or rat in a room or store, seeing them on TV or in movies, someone joking about them, or smelling them. Some people are grossed out by how mice and rats feel, while others are afraid of being nibbled on.
In many cases a phobic fear of mice is a socially induced conditioned response, combined with (and originated in) the startle response (a response to an unexpected stimulus) common in many animals, including humans, rather than a real disorder. At the same time, as is common with specific phobias, an occasional fright may give rise to abnormal anxiety that requires treatment.
Read more about this topic: Fear Of Mice