Varicella, or chickenpox, is one of the most common childhood
diseases. It is caused by the varicella-zoster virus. Most
people in the United States get chickenpox while they are still
children. Until the late 1990's there were about 4 million cases
a year. But now that people are using varicella vaccine, that
number has begun to dropVaricella vaccine is a live-virus
vaccine. It has been used in some parts of the world, such as
Japan, for over 20 years. It was licensed in the United States
in 1995.