Primary varicella is an endemic disease. Varicella is one of the classic diseases of childhood, with the highest prevalence occurring in the 4 - 10 years old age group.
Varicella is highly communicable, with an attack rate of 90% in close contacts.
Most people become infected before adulthood but 10% of young adults remain susceptible.
Herpes zoster, in contrast, occurs sporadically and evenly throughout the year.