Because chicken pox and shingles infections start out from different origin points, they are considered different diseases.  It is important to realize that when herpes zoster infects someone for the first time, it comes from an external source, and also has not made vast inroads into the nervous system. On the other hand, when it reactivates as shingles disease, the nervous system becomes the primary means of trasmission, as well as the source of the pathogen.