Blisters begin with redness in the spot where there is irritation and can be itchy. Eventually, a painful, bubble-like sac appears on the affected area. If the fluid sack is ruptured, the blister can become infected and begin to form pus. If the infection progresses, red streaks will form on the skin running upward from the site of the blister, indicating cellulitis. If a streak develops or you begin to run a fever, seek prompt medical treatment.