Chickenpox is a contagious illness caused by a virus called varicella-zoster virus. It usually starts with a fever, headache, and feeling tired. After a few days, small red bumps appear on the body, which can fill with fluid and become itchy. The bumps then scab over and eventually go away. Chickenpox can spread easily from person to person through the air when someone with the virus coughs or sneezes, or through direct contact with the fluid from the blisters.