Herpes zoster is a disease caused by the varicella-zoster virus infection. When blisters appear, if they turn black and become dry at the same time, they are considered to be improving. If the blisters turn black but do not dry out and become red around them, it is possible that the blisters are infected with bacteria, so it is not possible to determine whether there is improvement by the color of the blisters. Note that the blister must avoid scratching, and if the blister breaks avoid contact with water to avoid combined bacterial infections.