Shingles ( Herpes Zoster ) is a painful, and relatively long-lasting skin condition which affects around 1/3rd of the Western population at some stage in their lives. It's known to be associated with the 're activation' of the Varicella Zoster Virus (VZV) - which remains in a person's body after an infection of Chickenpox.
It's assumed that the VZ virus remains inactive in nerve cells - but has never been isolated from samples of living cells.
In addition, the physiological mechanism which triggers the re-emergence of the dormant virus, has not yet been discovered.
Further reading : Varicella in Centers for Disease Control and Prevention Epidemiology and Prevention of Vaccine-Preventable Diseases, 13th Edition