Chickenpox is a very contagious disease caused by the varicella-zoster virus. Chickenpox causes an itchy rash that scabs over and usually lasts about a week. Scratching the scabs can cause dangerous skin infections and permanent scarring. Chickenpox can also cause fever, tiredness, loss of appetite, headache, and even death. A person who has had chickenpox can develop shingles later in life.