Chickenpox (varicella) and shingles (herpes zoster) are two different forms of disease caused by the varicella-zoster virus, which is a deoxyribonucleic acid (DNA) virus closely related to herpes simplex and Epstein-Barr viruses. Initial infection causes varicella, a common childhood infection characterized by fever, malaise, and a rash consisting of dozens to hundreds of small fluid-filled lesions (vesicles) that are individually surrounded by reddened skin. Successive crops of lesions appear that eventually ulcerate and crust over during the two-week course of the disease. The virus is spread from person to person by the highly infectious respiratory secretions and lesion drainage. Varicella is rarely a serious disease in normal children but can be severe in immunocompromised individuals or in the rare adult who escaped childhood infection. Primary infection results in immunity to a new varicella-zoster virus, but the original virus lies dormant in nerve ganglia cells.