A viral infection caused by varicella (reactivated chickenpox virus). After recovery from chickenpox, some virus particles continue to survive for decades. This surviving virus lives in clusters of nerve cells called nerve roots where it is protected from your immune system. The virus re-emerges many years later causing painful lesions that look like pimples. They typically appear on only one side of the body and are localized to one area. Stress, old age and other infections can cause the re-emergence of the virus. Also called herpes zoster.