MODEL RELEASED. Chickenpox. Young girl with chickenpox (or varicella). This is a common infectious disease of childhood caused by the varicella-zoster virus. It causes headache and fever followed by eruption of a rash over most of the body. The rash appears in crops and consists of clusters of small,red,itchy spots that become fluid-filled blisters within a few hours. It is rare to have the disease twice, as a childhood attack confers lofelong immunity. While the disease is not serious in children in adults it can be dangerous.