Bell’s palsy is a condition that causes temporary facial paralysis (palsy). It usually only affects the muscles on one side of your face. You may have a lopsided smile or an eyelid that you can’t fully close. It rarely affects both sides of your face. Bell’s palsy happens when there’s inflammation and swelling of your seventh cranial nerve — the nerve that controls facial muscles. Certain conditions (like viral infections) can cause inflammation, but many cases of Bell’s palsy have no clear cause (idiopathic). Bell’s palsy can affect anyone at any age. But it’s most likely to affect people between the ages of 15 and 60. The average age of onset is 40 years.