Chickenpox, as defined by the Centers for Disease Control & Prevention (CDC), "is a very contagious disease caused by the varicella zoster virus (VZV). It causes a blister-like rash, itching, tiredness, and fever. The rash appears first on the stomach, back and face and can spread over the entire body causing between 250 and 500 itchy blisters. Chickenpox can be serious, especially in babies, adults, and people with weakened immune systems. The best way to prevent chickenpox is to get the chickenpox vaccine."