What causes the virus Shingles?

Varicella, the same virus that causes chickenpox. It infects children in the form of acute chickenpox and then lies latent in the cells of people. Later on in their lives, an event happens, lets say immunosuppression, chemotherapy, organ failure, some kind of event that forces the body's immune system to shut down. At that point, the body becomes vulnerable and immunocompromised, causing the virus to resurface in the form of an opportunistic infection called shingles.