Chickenpox is a highly contagious viral disease caused by the varicella-zoster virus (VZV) and characterized by hundreds of itchy, fluid-filled blisters on the skin that burst and form crusts. The vast majority of the time the disease affects children, though adults may become infected as well.

Chickenpox is transmitted by direct contact between a rash and an open sore, or through the inhalation of contaminated droplets in air or moisture. These droplets are usually spread by coughing or sneezing.  Millions of cases occur in the United States each year, though the overall prevalence of the disease has decreased substantially since the development of a preventative vaccine in 1995. Prevalence of the disease is seasonal, with peak outbreaks occurring in late winter or spring.

Chickenpox is highly contagious. In a household, those without immunity will likely become infected if they come into contact with an infected person. Airborne droplets are the most common way that chickenpox is spread, though direct contact with blister fluid from an infected individual is also a common cause. An infected individual has the potential to infect others until all of the fluid-filled blisters (macules) have burst and crusted over.

After the chickenpox virus first enters the body, it reproduces in the lymph nodes for about two to four days before it enters the bloodstream and spreads to other parts of the body. After the virus has reproduced in certain vital organs (e.g., spleen, liver), it is spread to the nerves of the spine. Here, the virus waits to be activated. Activation of VZV may occur spontaneously or in reaction to certain events (e.g., stress, fever, weakened immune system). Pimple-like blisters (similar to dew drops on a flower petal) or a rash typically become apparent within two weeks after exposure. However, symptoms can appear anywhere from 10 to 21 days following exposure.

A person infected with chickenpox is contagious from up to 48 hours before the initial rash occurs until all blisters have burst and crusted over. Bacterial infection of lesions followed by scarring are the most common complications of the disease.

Most people who are either vaccinated or who have had chickenpox once develop lifetime immunity to the disease. However, chickenpox reinfection may occur in a very small percentage of individuals.

There is no reliable way to predict how serious a chickenpox infection may be, regardless of whether the individual has been vaccinated or not. Individuals who are not vaccinated or who have compromised immune systems (e.g., pregnant women, Human Immunodeficiency Virus/Acquired Immune Deficiency Syndrome) may be susceptible to a more serious, sometimes fatal form of the disease.

People exposed to VZV may also develop herpes zoster (shingles) later in adulthood. This condition is characterized by brief outbreaks of painful blisters. Shingles may also lead to postherpetic neuralgia in some instances, a condition where the pain of shingles remains long after the blisters go away.