Chicken pox is a highly contagious viral infection. It usually starts with skin rash, mainly on the torso and head rather than the extremities, and becomes itchy and raw pockmarks, which mostly heal without scarring. A subject who has had chicken pox has the virus in a dormant form in the body's nerve tissues, but it can be reactivated in later life in a different form known as shingles (herpes zoster).