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Varicella-zoster virus (VZV), a close but distinct relative of the other seven human herpesviruses, including herpes simplex virus (HSV), causes two diseases. Varicella (chickenpox), a generalized illness, is its primary infection, and zoster (shingles) is its secondary infection, caused by reactivation of VZV from latency. Varicella infection occurs in almost all people over their lifetimes. VZV becomes latent after varicella and usually persists silently and indefinitely. VZV reactivates, however, to cause zoster in roughly 20% of individuals, with higher reactivation rates in immunocompromised patients and the elderly.

In the prevaccine era in the United States prior to 1995, approximately 4 million cases of varicella and 1 million cases of zoster occurred annually. Varicella was primarily a disease of children younger than age 10 years and zoster an illness of adulthood. Childhood varicella infection, however, is less common than adult infection in...

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