Bell palsy

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Bell palsy (a type of facial nerve palsy) is sudden weakness or paralysis of muscles on one side of the face due to malfunction of the 7th cranial nerve (facial nerve). This nerve moves the facial muscles, stimulates the salivary and tear glands, enables the front two thirds of the tongue to detect tastes, and controls a muscle involved in hearing.

The cause may be a viral infection or an immune disorder that causes the facial nerve to swell.
People may feel pain behind the ear, then one side of the face may become weak or completely paralyzed, and they may be unable to taste with the front of the tongue on the affected side.
Doctors usually base the diagnosis on symptoms.
Corticosteroids, which are used to reduce swelling of the nerve, help people recover and regain facial movement slightly more quickly.
With or without treatment, most people recover completely within several months.

(See also Overview of the Cranial Nerves.)
Bell palsy is a type of facial nerve palsy, which was originally thought to have no identifiable cause (idiopathic facial nerve palsy). However, current evidence suggests that in some cases, Bell palsy can be caused by a viral infection or another disorder.
Evidence suggest that common causes of Bell palsy include

Infection by herpes simplex virus type 1 (which causes mouth infections, such as cold sores)
Shingles

Other viruses, such as coxsackievirus, cytomegalovirus, and the viruses that cause mumps, rubella, mononucleosis, or influenza, may also cause Bell palsy.
Infection causes the nerve to swell. When the nerve is swollen, it is squeezed (compressed) by the narrow passageways in the skull that it passes through.
Other disorders can cause facial nerve palsy. Lyme disease can cause facial nerve palsy that, unlike Bell palsy, may affect both sides of the face. In African-Americans, sarcoidosis is a common cause of facial nerve palsy. Sometimes tumors and skull fractures cause facial palsy.