highly contagious infectious disease caused by the varicella-zoster virus (Herpesvirus 3); usually affects children, is spread by direct contact or respiratory route via droplet nuclei, and is characterized by the appearance on the skin and mucous membranes of successive crops of typical pruritic vesicular lesions that are easily broken and become scabbed; chickenpox is relatively benign in children, but may be complicated by pneumonia and encephalitis in adults.