Chickenpox is a common childhood disease, and is very contagious. The affected child or adult may develop hundreds of itchy, fluid-filled blisters that burst and form crusts. Chickenpox is caused by a virus. The virus that causes chickenpox is varicella-zoster, a member of the herpesvirus family. The same virus also causes herpes zoster (shingles) in adults.
Most cases of chickenpox occur in children younger than ten. The disease is usually not serious, although serious complications sometimes occur. Adults and older children are generally affected more harshly than younger children. A chickenpox vaccine is part of the routine immunization schedule. It is about 100% effective against moderate or severe illness, and 85-90% effective against mild chickenpox. The chickenpox vaccine is the only routine vaccine that does not require a booster.