Chickenpox blisters on a woman's back. Chickenpox, or varicella, is caused by infection with the herpesvirus varicella-zoster. It is common in children but, because infection confers lifelong immunity to the disease, rare in adults. Symptoms include a mild fever followed by the development of a rash of small red itchy spots over most of the body. The spots become fluid-filled blisters within a few hours.