Shingles is an acute infection of the peripheral nervous system. It is caused by herpes zoster. This is the virus that causes chickenpox and after a person recovers, the virus retreats to a posterior (dorsal) root ganglion. If the virus is reactivated, the immune system usually prevents it from spreading.
However, from time to time, the rekindled virus overcomes a weakened immune system, leaves the ganglion and travels down sensory neurons by fast axonal transport. The result is pain, discoloration of the skin, and a characteristic line of skin blisters. The line of blisters marks the distribution (dermatome) of a particular cutaneous sensory nerve).