Chickenpox is caused by the herpes zoster virus, also known as the varicella zoster virus. It is spread by droplets from a sneeze orcough, or by contact with the clothing, bed linens, or oozing blisters of an infected person. The onset of symptoms occurs seven to 21 days after exposure. The disease is most contagious a day before therash appears and up to seven days after, or until the rash is completely dry and scabbed.