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.