Answer:
Bell's Palsy is a viral infection caused by the Herpes virus; the same virus that causes chicken pox and shingles. This viruslays dormant in your system in the root ganglia behind your face (this is why it is hard to get chicken pox twice because your immune system constantly has antibodies ready to target and attack virally-infected cells). This virus will travel through your body and cause a huge variety of diseases/disorders during any immuno-compromised condition i.e. stress, fatigue, other illness, etc.
Bell's Palsy is one of these conditions; it is mostly harmless as it's only symptom is numbness on one side of the face in all of the tri-geminal nerves (forehead, mid-face and lower face). The only real complication can come from the fact that your eye might not close all of the way during sleep. This can dry out the cornea which can be extremely painful and cause irreparable damage, resulting in requiring a cornea transplant. Usually an antiviral medication, like acyclovir, some moisturizing drops and an eye patch to keep the eye protected will be prescribed to treat this condition and the numbness will go away in time. Depending on your age, this can take up to 6 months for full recovery.