Chicken pox is a virus that causes an itchy rash but typically does not have any major or lasting effects on an infected person. Before 1995, chicken pox parties were popular. In a chicken pox party, healthy children were brought around children who were ill with chickenpox so that they would be exposed to the virus and their immune system would learn to fight it. A vaccine for chickenpox was developed in 1995, and chicken pox parties are now rare.