How does shingles occur?

Most people have chickenpox at some stage (usually as a child). The virus does not completely go after you have chickenpox. Some virus particles 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 re-activated virus travels along the nerve to the skin to cause shingles.

In most cases, an episode of shingles occurs for no apparent reason. Sometimes a period of stress or illness seems to trigger it. A minor ageing of the immune system may account for it being more common in older people. (The immune system keeps the virus inactive and prevents it from multiplying. A slight weakening of the immune system in older people may account for the virus reactivating and multiplying to cause shingles.)

Shingles is also more common in people with a poor immune system. For example, shingles commonly occurs in younger people who have HIV, AIDS or whose immune system is suppressed with treatment such as steroids or chemotherapy.