Chickenpox (varicella-zoster virus or VSV) is highly contagious to people who have not previously had the disease or who have not been vaccinated. It is transmitted by direct contact with a rash or through contaminated droplets in air or moisture, usually spread by coughing or sneezing.

Chickenpox infections spread quickly to non-immune individuals in places where there is a lot of interpersonal contact (e.g., child-care facilities, schools). Age is a risk factor, as the majority of chickenpox infections occur in children or adolescents. Children who have chickenpox at a very young age may not build up enough antibodies to prevent a second infection later in life.

The rate of complications and fatalities is substantially higher for infected individuals older than age 15 or younger than age one when left untreated. Furthermore, chickenpox in adults is typically more severe and includes a greater number of blisters.

Although highly effective, the protection provided by the chickenpox vaccine appears to decrease somewhat over time. Studies have found that the risk of contracting chickenpox in vaccinated people rises over a period of eight years, and that infection is relatively common. However, in many cases the symptoms are milder.

When vaccinated, healthy children rarely transmit the vaccine virus to susceptible people, but those with compromised immune systems are at higher risk of transmitting the disease.

Occasionally, serious complications result from chickenpox infections. As noted earlier, the risk of complications increases with age and in those with compromised immune systems (e.g., people who have undergone transplants, those with Human Immunodeficiency Virus/Acquired Immune Deficiency Syndrome).

The most common complication of chickenpox in healthy children under 5 years of age is bacterial infection (e.g., impetigo, cellulitis) due to frequent scratching of the lesions. Other potential (though rare) complications include:

• Varicella-pneumonia. The spread of the varicella virus in the bloodstream to the lungs may cause varicella-pneumonia in some patients.
  
  
• Necrotizing fascitis. A serious, painful infection of the membranes (fascia) that cover one or more muscles that may spread quickly to other parts of the body. Necrotizing fascitis is one of the most serious chickenpox complications. 
  
  
• Guillain-Barre syndrome. A disorder in which the body’s immune system attacks part of the nervous system.
  
  
• Meningoencephalitis. Inflammation of the brain, spinal cord and the protective membranes that cover each respective area (meninges).
  
  
• Reye’s syndrome. A rare, often fatal pediatric disorder characterized by vital organ damage (e.g., kidney, liver) that develops after chickenpox or other viral infections. Reye’s syndrome is associated with the use of aspirin in children.
  
  
• Arthritis. A disorder in which the body’s immune system attacks the joints, causing pain and inflammation.
  
  
• Neurologic complications. Chickenpox may rarely cause various temporary neurological disorders (e.g., acute cerebellar ataxia).  
  
  
• Liver problems. Serious complications involving the liver that rarely occur from chickenpox infection (e.g., hepatitis).

Pregnant women who are exposed to chickenpox during the first trimester may give a form of the disease to their fetus (fetal varicella). This may lead to problems with fetal development and other serious complications (e.g., low birth weight). Possible birth defects include shortening and scarring of the limbs, cataracts (a condition in which the lens of the eye becomes cloudy), small head size, abnormal brain development and mental retardation.

Chickenpox infections experienced during the final three weeks of pregnancy may cause the infant to be born with symptoms of the disease (e.g., rash, fever). However, the administration of chickenpox antibodies (e.g. acyclovir, varicella zoster immune globulin) within 24 hours following exposure to VZV is often effective at preventing chickenpox and related complications from occurring.

In some cases, people who have had chickenpox during childhood get a related adulthood disease called shingles (herpes zoster). Following a chickenpox infection, there may be remnants of varicella-zoster in nerve cells that reactivate themselves years later as shingles; a disease characterized by brief outbreaks of painful blisters. In addition, shingles may lead to postherpetic neuralgia, a condition where the pain of shingles remains long after the blisters go away.