Chickenpox is a mild and common childhood illness that most children catch at some point. It causes a rash of red, itchy spots that turn into fluid-filled blisters. They then crust over to form scabs, which eventually drop off. Some children have only a few spots, but other children can have spots that cover their entire body. These are most likely to appear on the face, ears and scalp, under the arms, on the chest and belly, and on the arms and legs.