Bell's palsy is a temporary facial paralysis (typically lasting 6 weeks to 6 months) that results from damage to the nerves which control facial muscles. It's usually one-sided, and has been estimated to affect around 1.5% of the global population at some point in their lives.

Bell's palsy occurs when the nerve that controls the facial muscles is swollen, inflamed, or compressed, resulting in facial weakness or paralysis. Exactly what causes this damage, however, is unknown.
Most scientists believe that a viral infection such as viral meningitis or the common cold sore virus—herpes simplex—causes the disorder. They believe that the facial nerve swells and becomes inflamed in reaction to the infection, causing pressure within the Fallopian canal and leading to ischemia (the restriction of blood and oxygen to the nerve cells). In some mild cases (where recovery is rapid), there is damage only to the myelin sheath of the nerve. The myelin sheath is the fatty covering-which acts as an insulator-on nerve fibers in the brain.
The disorder has also been associated with influenza or a flu-like illness, headaches, chronic middle ear infection, high blood pressure, diabetes, sarcoidosis, tumors, Lyme disease, and trauma such as skull fracture or facial injury."

Source : US National Institute of Neurological Disorders and Stroke ^(archived)