Chickenpox, also known as varicella, is a very contagious rash illness caused by the varicella-zoster virus (VZV). VZV stays dormant in the body can reactivate later in life to cause shingles (herpes zoster). Chickenpox causes a fever and an itchy rash that starts on the torso and face and then spreads all over the body. The rash has fluid-filled blisters that last for 3 to 7 days before they scab. People with chickenpox are contagious from two days before the rash starts until the entire rash scabs over.