Chickenpox is a preventable illness caused by the varicella-zoster virus. It was very common before the varicella (chicken pox) vaccine was introduced. It is extremely contagious, but the rash that it produces is usually harmless and disappears on its own.
While you can get chickenpox at any age, most cases occur before the age of 14. Anyone who's had the disease once is usually immune for life, although they can still get shingles, a painful skin rash caused when the varicella-zoster virus becomes active again, even many years after they've had chickenpox.
Chickenpox usually occurs during the school year; it is less common during the summer months. Since the introduction of the chickenpox vaccine, many children never get chickenpox.