Chickenpox (varicella) is a contagious illness that causes an itchy rash and red spots or blisters (pox) all over the body.Chickenpox can cause problems for pregnant women, newborns, teens and adults, and people who have immune system problems that make it hard for the body to fight infection.hickenpox is caused by the varicella-zoster virus. It can spread easily. You can get it from an infected person who sneezes, coughs, or shares food or drinks. You can also get it if you touch the fluid from a chickenpox blister.