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What Is It?

Chickenpox is an infection that causes an itchy, blistering rash and is very contagious, meaning it is spread easily from one person to another. It is caused by varicella-zoster virus (VZV), which enters the body through the mouth and nose after contact with an infected person.

A person with chickenpox can spread the disease to someone else from one day before the rash appears until all chickenpox blisters have crusted over. Once someone has had a chickenpox infection, he or she almost always develops a lifelong immunity, meaning that person usually does not get chickenpox a second time. The exception is a child who is infected at a very young age. Young children usually have milder cases and may not build up enough protection against the disease. Therefore, these children may develop the disease again later in life.

Because chickenpox is so contagious, 90 percent of a patient's family also will develop the illness if they live in the same house and are not already immune. In the past, chickenpox cases often occurred in groups (epidemics), usually during the late winter and early spring. However, the number of cases of chickenpox has dropped dramatically because of the chickenpox (varicella) vaccine, which was licensed in 1995 and is recommended for all children.

Chickenpox is an uncomfortable infection that, in most cases, goes away by itself. However, chickenpox also has been associated with serious complications, including death. About one of every 100 children infected with chickenpox will develop a severe lung infection (pneumonia), an infection of the brain (encephalitis), or a problem with the liver. Dangerous skin infections also can occur. Before the introduction of the vaccine, about 100,000 people were hospitalized and 100 people in the United States died each year of chickenpox, most of them previously healthy children. Adolescents and adults who develop chickenpox are also at high risk of developing serious complications.

After a person has chickenpox, the virus typically lives silently in the nervous system of the body for the rest of a person's life. It may reactivate (come to life again) at any time when the body's immune defenses are weakened by stress or illness (such as cancer or HIV infection) or by medications that weaken the immune system. The most common reason for the virus to reactivate is getting older. Reactivation of the virus causes a condition called shingles, a painful blistering skin rash that typically occurs on the face, chest or back, in the same area where one or two of the body's sensory nerves travel.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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