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Because varicella is highly contagious (an active case of chickenpox is transmitted to about 90% of susceptible close contacts), infection occurs in nearly everyone during childhood. By the age of 20, only about 5% to 10% of people are seronegative. Varicella is usually a mild illness, but serious complications, such as encephalitis, purpura fulminans, and bacterial superinfection occasionally occur. The disease is generally more severe in adolescents and adults than in children. Varicella during pregnancy is especially hazardous: in the first 20 weeks it confers a 2% risk of congenital defects, in the last trimester it may lead to severe pneumonia, and in the 5 days before delivery to the 2 days after, maternal disease can cause serious, o…