http://www.medinfo.co.uk/conditions/shingles.html
Causes
You can only get shingles (Herpes Zoster) if you have previously had chickenpox. After having chickenpox the virus lies dormant in the nerves, and shingles occurs when it is revitalised in one particular nerve to the skin, thus explaining the way it affects a clearly demarcated band of skin only.
Usually the cause is a decrease in your body's natural resistance, which may come through other infections, stress, being generally run down, or occasionally, when the body's immune defences are affected by certain drugs or other immune deficiencies.
Prevention
You cannot do much to avoid shingles.
It is not caught from contact with someone with shingles or with chickenpox.
If you have shingles you should stay away from other people until the blisters have dried (usually about seven days), as there are virus particles in the blister fluid. The risk is that people who have not had chickenpox might catch that from you.
Those at particular risk are people whose immune system is not functioning normally (who are immunocompromised), as for example in people on steroids, chemotherapy for cancer or those with transplants or with AIDS, so do try to avoid contact with people in these categories if you have shingles.