Bell's palsy is a form of facial paralysis resulting from a dysfunction of the cranial nerve VII (the facial nerve) causing an inability to control facial muscles on the affected side. Often the eye in the affected side cannot be closed. The eye must be protected from drying up, or the cornea may be permanently damaged, resulting in impaired vision. In some cases denture wearers experience some discomfort. The common presentation of this condition is a rapid onset of partial or complete paralysis that often occurs overnight. In rare cases, it can occur on both sides resulting in total facial paralysis. Bell’s palsy is defined as a one-sided facial nerve paralysis of unknown cause. Several other conditions can also cause facial paralysis, e.g., brain tumor, stroke, myasthenia gravis, and Lyme disease; however, if no specific cause can be identified, the condition is known as Bell's palsy. It is thought that an inflammatory condition leads to swelling of the facial nerve. The nerve travels through the skull in a narrow bone canal beneath the ear. Nerve swelling and compression in the narrow bone canal are thought to lead to nerve inhibition or damage.

Usually it gets better on its own with most people achieving normal or near-normal function. Corticosteroids have been found to improve outcomes, when used early, while anti-viral drugs have not.[3][4] Many show signs of improvement as early as 10 days after the onset, even without treatment.

Bell's palsy is the most common acute disease involving a single nerve and is the most common cause of acute facial nerve paralysis (>80%). It is named after Scottish anatomist and Edinburgh graduate Charles Bell (1774–1842), who first described it. It is more common in adults than children.
Bell's palsy is characterized by a one-sided facial droop that comes on within 72 hours.
The facial nerve controls a number of functions, such as blinking and closing the eyes, smiling, frowning, lacrimation, salivation, flaring nostrils and raising eyebrows. It also carries taste sensations from the anterior two-thirds of the tongue, via the chorda tympani nerve (a branch of the facial nerve). Because of this, people with Bell's palsy may present with loss of taste sensation in the anterior 2/3 of the tongue on the affected side
Although the facial nerve innervates the stapedii muscles of the middle ear (via the tympanic branch), sound sensitivity and discuses are hardly ever clinically evident.
Although defined as a mononeuritis (involving only one nerve), people diagnosed with Bell’s palsy may have "myriad neurological symptoms" including "facial tingling, moderate or severe headache/neck pain, memory problems, balance problems, ipsilateral limb paresthesia’s, ipsilateral limb weakness, and a sense of clumsiness" that are "unexplained by facial nerve dysfunction
Some viruses are thought to establish a persistent (or latent) infection without symptoms, e.g., the varicella-zoster virus and Epstein-Barr viruses, both of the herpes family. Reactivation of an existing (dormant) viral infection has been suggested . as a cause of acute Bell's palsy. Studies suggest that this new activation could be preceded by trauma, environmental factors, and metabolic or emotional disorders, thus suggesting that a host of different conditions may trigger reactivation
Once the facial paralysis sets in, many people may mistake it as a symptom of a stroke; however, there are a few subtle differences. A stroke will usually cause a few additional symptoms, such as numbness or weakness in the arms and legs. And unlike Bell's palsy, a stroke will usually let patients control the upper part of their faces. A person with a stroke will usually have some wrinkling of their forehead. One disease that may be difficult to exclude in the differential diagnosis is involvement of the facial nerve in infections with the herpes zoster virus. The major differences in this condition are the presence of small blisters, or vesicles, on the external ear and hearing disturbances, but these findings may occasionally be lacking (zoster sine herpete). Reactivation of existing herpes zoster infection leading to facial paralysis in a Bell's palsy type pattern is known as Ramsay Hunt syndrome type 2.Lyme disease may produce facial palsy.[12] Sometimes the facial palsy occurs at the same time as the classic erythema migrans rash.[12] Other times, it occurs later.[12] In areas where Lyme disease is common, it may be the cause of facial palsy in half of cases.