ExRx.net

Well, the virus that causes chicken pox goes undercover in the nerve roots after the episode of chicken pox. Sometime later, and for UNKNOWN reasons, the virus becomes active again, causing shingles. To the best of my knowledge, the association with stress is very slight, if real at all. Many factors that cause immunodeficiency are known to increase the rate of activation, but the factors that trigger the event are not known. Stress is often mentioned, but the honest answer is "we don't know." Most people who get shingles have experienced stress recently, but most people who don't get shingles have experienced stress recently.

Sorry to hijack. It's a pet peeve of mine, doctors repeating myths like this.