Chickenpox is an illness caused by a virus -- specifically, the varicella zoster virus. The most characteristic symptom is a rash that can cover the entire body, producing up to 500 itchy blisters. Eventually, the blisters crust over. Other signs can include fever, diarrhea, and headache.
In general, chickenpox is treated at home, with the symptoms managed while the body fights the infection. In 1 out of 10 children with the illness, however, complications may result that can lead to a visit to the healthcare provider. Such complications include certain infections and pneumonia. A person with chickenpox is contagious one to two days before the rash appears and until all blisters have formed scabs.
(Click Chickenpox for more information on this illness, including how it is transmitted, how long the incubation period lasts, and whether a vaccine is available.)