Varicella occurs throughout the world but infection occurs at a younger age in temperate zones compared to the tropics. In temperate regions more than 90% of adults are immune to varicella while in tropical countries only 60% of adults are immune to it. Transmission is via the respiratory route and less commonly by direct contact with the lesions. A susceptible person may develop varicella following exposure to the lesions of herpes zoster. The severity of the disease is age-dependent, with adults having more severe disease and more complications. Varicella confers lasting immunity and second attacks are uncommon, especially in immunologically healthy people, but clinical reinfection with a mild varicella-like illness occasionally takes place.
The distinctive features of varicella are the centripetal distribution, the polymorphism in each affected site and the rapid progression of the individual lesion from vesicle to crust. Vesicles are common in the mouth, especially on the palate, and are occasionally seen on other mucous membranes, including the conjunctiva and genitalia. On the anal mucosa they may be followed by painful ulcers.