Shingles strikes about 1 million people a year, and the risk increases with age. It is caused by the varicella virus, which causes chickenpox, usually in childhood. The virus then lies dormant, often for decades, hiding in nerves along the spinal cord.
"For some reason the virus reactivates and travels from one nerve cell down that nerve and ends up on the surface of the body," says Rafael Harpaz, a medical epidemiologist at the National Center for Immunization and Respiratory Diseases, part of the Centers for Disease Control and Prevention.
Anyone who has had chickenpox is at risk, Harpaz says. Shingles causes a rash and blistering in a band along the nerve path, often on one side of the body. The pain lasts a week or longer.