The varicella zoster virus causes both chickenpox (varicella) and shingles (herpes zoster). Chickenpox is highly contagious and presents with a characteristic vesicular rash. It is usually a mild childhood illness but can occasionally cause complications affecting the brain, lungs, liver or other organs. The virus becomes latent after the initial infection and can reactivate later in life to cause shingles. Diagnosis is based on signs and symptoms but can be confirmed by virus detection in lesions or blood tests showing antibodies to the virus.