What is Ramsay Hunt syndrome, the virus attacking Justin Bieber's face?

Pop star Justin Bieber announced his face is partially paralyzed by a viral condition called Ramsay Hunt syndrome,

Ramsay Hunt syndrome is a rare neurological disorder that occurs when the varicella-zoster virus infects a nerve in the head near the inner ear.

After having chickenpox as a child or shingles as an adult, the virus can lay dormant in the body.

Why the virus reactivates and produces symptoms of Ramsay Hunt is not known.