Shingles is caused by the same virus that is responsible for chicken pox (the varicella-zoster virus, a member of the Herpes family of viruses) and people who have had chicken pox are usually immune from getting it again.
However, the virus never truly leaves the body and can lie dormant in the spinal cord and nerve tissue for years and may reactivate and cause shingles.
Approximately 10-20% of people who have had chicken pox will suffer from Shingles with the risk increasing with age. Shingles most commonly occurs in those over 50.
Whilst not fully understood, it appears that situations affecting the immune system, such as illness, stress or trauma, can be a trigger for the virus to be re-activated. This condition is generally more common in people with weakened immune systems.