Shingles is an infection of a nerve and the surrounding skin surface that is supplied by the nerve, caused by the varicella-zoster virus. This is the same virus that causes chickenpox.
Most people have chickenpox at some stage (usually as a child), but for some people the virus does not completely clear after you have chickenpox. Some virus particles can remain inactive in the nerve roots next to your spinal cord. They do no harm there, and cause no symptoms. For reasons that are not clear, the virus may begin to multiply again (reactivate). This is often years later. The reactivated virus travels along the nerve to the skin to cause shingles.
Shingles and chickenpox are both caused by the varicella zoster virus. This virus is related to (but not the same as) the herpes viruses that cause cold sores and genital herpes, which is why shingles is sometimes called herpes zoster.
After someone has had chickenpox, the virus stays sleeping in that person’s nervous system for the rest of his or her life, even though the chickenpox goes away.
In many people, the virus will never come back. But in some people it flares up and causes shingles. It is possible to get shingles more than once, but this is not common.