Varicella-zoster virus (VZV) infection causes two clinically distinct forms of disease: varicella and herpes zoster. Primary VZV infection results in the diffuse vesicular rash of varicella, or chickenpox. Whereas chickenpox is a mild disease in otherwise healthy children, varicella virus in an immunodeficient individual may lead to a more serious form. The epidemiology of varicella has changed dramatically since the introduction of the varicella vaccine in 1995. Two live, attenuated varicella zoster virus-vaccines are licensed in the United States for the prevention of chickenpox. Varicella vaccine is licensed for use in healthy children at 12 months of age.