Diseases & Conditions



Chickenpox (varicella) is a highly contagious infection with the varicella-zoster virus that produces a characteristic itchy rash, consisting of small, raised, blistered, or crusted spots.

Chickenpox is an infection that mostly affects children. Before the introduction of a vaccine in 1995, about 90% of children developed chickenpox by age 15. Now, the use of the vaccine has decreased the number of cases of chickenpox per year by about 70%. The disease is spread by airborne droplets of moisture containing the varicella-zoster virus. A person with chickenpox is most contagious just after symptoms start but remains contagious until the last blisters have crusted.

Although most people with chickenpox simply have sores on the skin and in the mouth, the virus sometimes infects the lungs, brain, heart, or joints. Such serious infections are more common in newborns, adults, and people with an impaired immune system.

A person who has had chickenpox develops immunity and cannot contract it again. However, the varicella-zoster virus remains dormant in the body after an initial infection with chickenpox, sometimes reactivating in later life, causing shingles. A new vaccine against varicella-zoster is available for older adults, which may decrease the risk of developing shingles in later life.

Symptoms and Diagnosis

Symptoms begin 11 to 15 days after infection. They include mild headache, moderate fever, loss of appetite, and a general feeling of illness (malaise). Younger children often do not have these symptoms, but symptoms are often severe in adults.

About 24 to 36 hours after the first symptoms begin, a rash of small, flat, red spots appears. The spots usually begin on the trunk and face, later appearing on the arms and legs. Some children have only a few spots. Others have them almost everywhere, including on the scalp and inside the mouth. Within 6 to 8 hours, each spot becomes raised; forms an itchy, round, fluid-filled blister against a red background; and finally crusts. Spots continue to develop and crust for several days. The spots may become infected by bacteria (see Bacterial Skin Infections: Introduction ), causing erysipelas, pyoderma, cellulitis, or bullous impetigo. New spots usually stop appearing by the fifth day, the majority are crusted by the sixth day, and most disappear in fewer than 20 days.


Spots in the mouth quickly rupture and form raw sores (ulcers), which often make swallowing painful. Raw sores may also occur on the eyelids and in the upper airways, rectum, and vagina. Spots in the voice box (larynx) and upper airways may occasionally cause severe difficulty in breathing. Lymph nodes at the side of the neck may become enlarged and tender. The worst part of the illness usually lasts 4 to 7 days.

Lung infection occurs in about 1 out of 400 people, especially adolescents and adults, resulting in cough and difficulty breathing. Brain infection (encephalitis) is less common and produces unsteadiness in walking, headache, dizziness, confusion, and seizures. Heart infection sometimes causes a heart murmur. Joint inflammation produces joint pain. Inflammation of the liver and problems with bleeding may also occur.

Reye's syndrome, a rare but very severe complication that occurs almost only in those younger than 18, may begin 3 to 8 days after the rash begins.

A doctor is usually certain of the diagnosis of chickenpox because the rash and the other symptoms are so typical. Measurement of the levels of antibodies in the blood and laboratory identification of the virus are rarely needed.


In the United States, children are routinely vaccinated against varicella-zoster beginning at 12 months of age, with a second dose given at 4 to 6 years of age (see Vaccinating Infants and Children ). Older children without immunity also may be vaccinated. Susceptible people who are at high risk of complications—such as those with an impaired immune system and pregnant women—and who have been exposed to someone with chickenpox—may be given antibodies against the varicella virus (varicella-zoster immune globulin). Isolation of an infected person helps prevent the spread of infection to people who have not had chickenpox. Children should not return to school and adults should not return to work until the final blisters have crusted.

Prognosis and Treatment

Healthy children nearly always recover from chickenpox without problems—only about 2 out of 100,000 children die. However, even this low rate means that before routine immunization, 100 children died annually in the United States because of complications of chickenpox. The infection is more severe in adults, of whom about 30 out of 100,000 die. Chickenpox is fatal in up to 15% of people with an impaired immune system.

Mild cases of chickenpox require only the treatment of symptoms. Wet compresses on the skin help soothe itching, which may be intense, and prevent scratching, which may spread the infection and cause scars. Because of the risk of bacterial infection, the skin is bathed often with soap and water, the hands are kept clean, the nails are clipped to minimize scratching, and clothing is kept clean and dry. Drugs that relieve itching, such as antihistamines, are sometimes given by mouth. If a bacterial infection develops, antibiotics may be needed.

Doctors may prescribe antiviral drugs, such as acyclovir Some Trade Names ZOVIRAX , valacyclovir Some Trade Names VALTREX , and famciclovir Some Trade Names FAMVIR , for adolescents and adults as well as for groups at high risk of complications, such as people with skin disorders like eczema, people who are being treated with aspirin Some Trade Names ECOTRIN ASPERGUM or corticosteroids, premature infants, and children with immune system disorders. The drugs must be given within 24 hours of onset of disease to be effective. These antiviral drugs are usually not given to pregnant women.

Last full review/revision May 2007 by Mary T. Caserta, MD

Source: The Merck Manual Home Edition