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.