Chickenpox is contagious from 1-2 days before the onset of symptoms until the chickenpox rash crusts over and falls off. So the chickenpox rash is not contagious after it crusts over and falls off. However, chickenpox is a highly contagious acute infectious disease that occurs mostly in children, so once a susceptible child is exposed to the chickenpox virus, 90% of children can develop the disease. Therefore, for safety reasons, even if the rash is completely crusted over and falls off, the child should be isolated until 21 days after the onset of the disease before he or she is allowed to enter daycare or school again.