Skin Rash-Pityriasis rosea

My 17 year old son showed me the strange rash that was developing on his chest and back.  It was itchy and each day seemed to be getting worse and worse.  The rash spread around his back and under the arms but spared his face and arms and legs.  After examining the pattern, I was sure of the diagnosis...Pityriasis rosea.

This strange rash occurs in the spring and the fall and mainly strikes