Bell's Palsy is usually a type of temporary sudden paralysis that causes weakness of the muscles of the face on one side. Rarely it can affect both sides. The facial nerve that supplies the muscles of the face is affected by the palsy. This nerve is called the facial nerve (the seventh of the twelve nerves that supply the face and neck regions).

It is one of the most common problems that affects the cranial nerves and is also the most common cause of facial paralysis all over the world.

Leonardo da Vinci's Mona Lisa has fascinated many with her enigmatic smile. Her smile has been the subject of intense debate for centuries. One theory put forward in 1989 suggested that the famous expression was the result of changes in facial muscles - partial degeneration followed by regeneration - that occur after Bell's palsy.