The Minnesota Department of Health states the chickenpox virus spreads by direct contact or through the air, with onset of the illness within two to three weeks of exposure and the subject contagious from one to two days before the lesions appear, eventually forming crusts or scabs. The virus then moves into the nerves where it sometimes reappears as shingles in the elderly, according to In Vivo.
About.com says the first signs of chicken pox often appear on the trunk of the body before spreading to the face and extremities. Lesions are normally between 1/8 and 1/4 inch in diameter and develop an irregular, rose petal outline. They then develop a characteristic thin blister-like appearance over the redness. The fluid grows cloudy after eight hours, and the skin breaks leaving a crust. This fluid contains the virus and infects most people who have not already had the virus or an inoculation. Once a crust forms, the lesion is no longer contagious. Crusts drop off after approximately a week, sometimes leaving scars. New lesions continue cropping up and moving through this cycle for several days after the first appearance. Children are permitted to return to school when all lesions have formed crusts and they are no longer contagious.