Chickenpox is an infection caused by varicella virus. The incubation period is 10 to 20 days. It starts with 1 to 3 days of symptoms resembling that of common cold: fever and sore throat. A rash appears afterwards, mainly on the trunk, face, and scalp. There is intense itching associated with the rashes. Seven days after the initial event, eruption of rashes stops. Once crusting begins, the patient is not contagious. Treatment is supportive: rehydration, and control of fever and itching. Recovery from an infection confers immunity in nearly all patients from future chickenpox infections. But, in about 10% of these patients, the varicella virus stays in sensory ganglia and reappears as herpes zoster.