MODEL RELEASED. Healing chickenpox lesions in a 5-year-old boy. Chickenpox (varicella) is a common infectious disease of childhood caused by the varicella-zoster virus. It causes headache and fever, followed by the 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 lifelong immunity. While the disease is not serious in children, in adults it can be dangerous.