Unless the immune system is compromised, it suppresses reactivation of the virus and prevents shingles outbreaks. Why this suppression sometimes fails is poorly understood,  but shingles is more likely to occur in people whose immune systems are impaired due to aging, immunosuppressive therapy , psychological stress , or other factors.   Upon reactivation, the virus replicates in neuronal cell bodies, and virions are shed from the cells and carried down the axons to the area of skin innervated by that ganglion. In the skin, the virus causes local inflammation and blistering. The short- and long-term pain caused by shingles outbreaks originates from inflammation of affected nerves due to the widespread growth of the virus in those areas. 
Post-herpetic neuralgia is defined as persistence or recurrence of pain in the same area, more than a month after the onset of herpes zoster. It becomes increasingly common with age, affecting about a third of patients over 40. It is particularly likely if there is facial infection . Post-herpetic neuralgia may be a continuous burning sensation with increased sensitivity in the affected areas or a spasmodic shooting pain. The overlying skin is often numb or exquisitely sensitive to touch. Sometimes, instead of pain, the neuralgia results in a persistent itch ( neuropathic pruritus ).