Maggie, great for clearing that up...I had forgotten they only recommend over 60.
Hopefully it is clear now that the virus resides in a nerve root near the spine and just suddenly (they don't know why) it decides to irritate and infect that particular nerve and thus the pain and blistering along only that one root pathway.
If it happens to be on the head, neck or face, it is an IMMEDIATE visit to an ophthalmologist. One could loose their vision.
I'm always comfortable referring people to the Mayo web site for more information so here it is.
Lynn
http://www.mayoclinic.com/health/shingles/DS00098