Shingles are caused by varicella-zoster, the same virus that causes chickenpox. If you've ever had chickenpox (typically during childhood), this virus is quietly hiding out in the roots of your nerves. It can reactivate and cause a painful skin rash. This is known as shingles or herpes zoster.
Not everyone who has had chickenpox will develop a case of shingles. Approximately 1 in 3 North Americans develop shingles at some point in their life. Most people who get shingles are 50 years of age or older, but it can happen at any age.