Chickenpox is a highly contagious viral disease that causes hundreds of itchy, fluid-filled blisters on the skin that burst and form crusts. The disease usually affects children, though adults may become infected as well. It is transmitted by direct contact with a rash or through contaminated droplets in air or moisture, usually spread by coughing or sneezing. The varicella-zoster virus (VZV) responsible for chickenpox is one of eight herpes viruses known to infect humans and is found throughout the world.

The overall prevalence of the disease has decreased substantially since the development of a preventative vaccine in 1995, though millions of people are still infected annually. Chickenpox is usually mild and does not cause any lasting complications, though it may be serious or fatal in rare cases. Pregnant women who are exposed to the disease during the first trimester may give a form of chickenpox to their fetus (fetal varicella).

A rash composed of numerous red, pimple-like blisters is the most obvious symptom of the disease. It usually appears as superficial spots about two to four days after the appearance of certain non-specific symptoms (e.g., fever, loss of appetite). These spots typically first appear in the torso area and spread to the head, face and limbs, eventually becoming blisters.

The number of blisters can range from a few to several hundred depending on the age and response of the patient to the virus. The spots redden and enlarge to about ¼ to ½ of an inch in diameter (6.35 to 12.5 millimeters) as they change to blisters. After several days, the blisters mature and break open, forming sores that eventually crust over. Most blisters heal within about two weeks without scarring.

The disease can usually be diagnosed by taking a patient’s complete medical history and performing a physical examination, with further testing often unnecessary. Mild cases of the disease typically only require the treatment of symptoms (e.g., rash, itching, fever). For more serious chickenpox infections, doctors may prescribe medications containing antibodies to the virus to shorten the duration of the disease and minimize the risk for complications.

Children are routinely vaccinated for the VZV beginning at about 12 months of age. Older individuals without immunity who have never contracted the disease may also be immunized. Chickenpox vaccine (varicella) is nearly always safe and effective. When administered correctly it is expected to be effective at preventing infection for between 10 to 20 years. The vaccine is not recommended for pregnant women, people with compromised immune systems (e.g., Human Immunodeficiency Virus/Acquired Immune Deficiency Syndrome) and people with certain allergies. Isolation from infected individuals with the disease is also a common and effective preventative measure.

Chickenpox reinfection does occasionally occur in a very small percentage of individuals. However, most people who are vaccinated or who have had the chickenpox once develop lifetime immunity to the disease.