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Shingles (also called herpes zoster) occurs as a result of a reactivation of the varicella-zoster virus (VZV) that causes chickenpox. Since the virus never completely leaves the body, it can reactivate in the form of shingles, causing localized rash and nerve irritation. Shingles typically occurs in older people and in people whose immune systems are weakened.
The immune system becomes less efficient with age. It is believed that this is the reason that shingles is more common in the elderly than in younger people, but the exact reason why the virus begins to start reproducing again is unknown. Stress can also weaken the immune system. Studies have shown that persons experiencing significant psychological stress are more prone to infection than those who are not under stress. For this reason, stress can also precipitate outbreaks of shingles in some people.
"Clinical manifestations of varicella-zoster virus infection: Herpes zoster"