Varicella-zoster virus?

Why do chickenpox have a really bad effect on adults and why are they mild for children?

If an adult gets chicken pox, it is usually worse than when a child gets it. Usually after a kid gets chicken pox, he will not get it again: he is immune. An adult who has never had chicken pox is not immune and can therefore get sick. Also, if a person has a second occurrence of chicken pox, the second time is usually worse than the first. It all has to do with your body's immunity to antigens on virally infected cells.