Mike,
Shingles (Herpes zoster) are caused by a virus that sits dormant in a nerve root and out of the blue when conditions a right expresses itself. The following website has some good info on shingles; http://en.wikipedia.org/wiki/Herpes_zoster
It can be a painful condition, but is self limiting and will resolve on its own without treatment. Usually medication and especially pain medication is needed to make the condition more tolerable.

Cheers,

Mike