My point is that I most certainly did not say that having chicken pox prevents shingles. Quite the opposite.
The chickenpox virus and the shingles virus are essentially the same virus.
What I said was that you cannot catch shingles from someone who has shingles.
You can, however, catch chickenpox from someone who has shingles if you have not had chickenpox before.
Once you have had chickenpox the virus stays in your system, alters slightly in some way and can return later as shingles, if, for example your immune system is compromised. You don't 'catch ' shingles, you just develop it.
Is this clear now ?
If it is not, read one of the excellent links posted by society in the first answer to this post.