Chicken pox most commonly occurs in children younger than ten years of age, though infection has recently become more common in adolescents and young adults. Infection is a manifestation of primary infection caused by the varicella-zoster virus. It is most common during late winter and early spring. Chickenpox is characterized by a generalized exanthem consisting of vesicles that occur in successive crops that begin on the face or scalp and spread rapidly to the trunk, rarely spreading to the extremities. The vesicles quickly evolve into pustules, crusts, and scabs. After healing, some residual poc mark scarring is commonly apparent. At this time,the virus persists in its latent form in the nerve cells of the dorsal root ganglia. Contrary to the commonly held belief that varicella-zoster viral infection can only occur once, reactivated infection can occur once immunity wanes. Reactivated infection gives rise to zoster.
Varicella-zoster virus is found only in humans and is usually spread by direct contact. However, transmission via the respiratory route and via contact with zoster lesions is also possible. In-utero infection can also occur. Contagiousness peaks during the 1 to 2 days before the onset of the rash. However, contagiousness can last as long as five days after the onset of lesions. In immunocompromised patients with progressive varicella, contagiousness lasts throughout the rash eruption period.