contagious viral disease characterized by an eruption of vesicles (small blisters) on the skin. The disease usually occurs in epidemics (epidemic), and the infected persons are generally between two and six years old, although they can be of any age. The incubation period is about two weeks; there are practically no premonitory symptoms, though slight fever for about 24 hours may precede the eruption. A number of raised, itching red papules appear on the back or chest; within 12 to 24 hours these develop into tense vesicles filled with a clear fluid, which in another 36 hours or so becomes opalescent. During the fourth day the vesicles shrivel up and become scabs. Eventually the scabs fall off, usually leaving no scar. Fresh spots continue to appear during the first three days, so that at the end of that time they can be seen in all stages of growth and resolution. The eruption is most marked on areas covered by clothing, but it also occurs on the face and limbs and on the mucous membrane of the mouth and palate. The temperature of the infected person rarely rises above 39 °C (102 °F). The disease runs a favourable course in most cases and usually produces permanent immunity.

The illness requires no treatment; the patient should be isolated at home to prevent infection of others. It is particularly important that the patient avoid contact with persons who have suppressed immune systems, in whom chickenpox can be severe or fatal. The patient also should be prevented from scratching, which could lead to ulceration of the vesicles and scarring; common antipruritic skin medications such as calamine lotion may be helpful. Complications, although uncommon, include secondary invasion of the skin by bacteria and spread of those bacteria to virtually any organ system in the body.

Chickenpox is caused by the varicella-zoster virus, the same virus that causes herpes zoster (also called zoster, shingles, or zona), a localized eruption of large blisters. Chickenpox is the clinical reaction to a first exposure to the virus. After recovery, the virus may settle in a nerve ganglion and be reactivated many years later. The cause of reactivation is not known. Upon reactivation, the virus travels along a nerve to the skin and causes herpes zoster. In most cases the lesions of herpes zoster occur in the distribution of a dermatome, an area of skin associated with the branch of a single spinal nerve. For this reason, herpes zoster is usually unilateral (on one side of the body). Severe pain is felt in the area of the blisters, which last from three to five days and then subside on their own. A person with herpes zoster is able to infect another person with varicella-zoster virus, resulting in chickenpox, but the reverse does not occur, because herpes zoster is a result of reactivation of latent virus from a prior varicella infection. Specific antiviral agents may have a role in the treatment of herpes zoster.

Injections of zoster immune globulin (ZIG), a preparation made from the plasma of adults who have recently had herpes zoster, are sometimes given to prevent the development of chickenpox in exposed children. ZIG contains antibodies (antibody) to varicella-zoster virus and provides temporary protection against the virus. ZIG administration is usually reserved for children with leukemia or immune-deficiency disorders, for whom chickenpox poses significant risks. In 1995 a chickenpox vaccine prepared from a live, attenuated strain of the virus was approved for use in the United States.