It is estimated that over 4 million cases occur yearly. Secondary attack rates in households often reach > 85% in susceptible individuals. Nosocomial transmission has been well documented in hospitals. Sources for nosocomial exposure of patients and staff have included patients, health care personnel and visitors. Persons who have had chickenpox previously are immune and cannot be reinfected; such individuals need not worry about exposure to patients with chickenpox. Also, a person with a positive varicella antibody test (titer) is also immune to chickenpox.