What Causes Shingles?

My wife has tested positive for shingles. She never had blisters but has felt weak and had some neurologic symptoms consistent with the disease for two years. She has also suffered from lichen planus in the last year. She recently had a physical and has tested negative for lots of diseases including HIV. She is concerned that she has some underlying disorder that is making her prone to conditions that take advantage of a compormised immune system, but her blood tests show no immune difficiency. Do you think her concern that she has an underlying problem besides the lichen planus and the shingles is true?