What are pustules? Pustules are small, pus-filled sores located at the surface of the skin. They are most commonly seen in acne, although they can be a sign of any infection involving the skin. Pustules appear on the skin as small, raised, reddened areas that typically have a whitish center. They may or may not be painful or tender to the touch. Pustules most commonly occur on the face, chest and shoulders and in areas of increased sweating. Pus is caused by the breakdown of inflammatory cells produced by the body to fight infection. Typically, pus forms during the course of a bacterial infection. Although neutrophils (type of immune system cell) initially engulf and kill bacteria, they themselves are eventually broken down and become a major constituent of pus. All types of bacteria that cause disease are capable of producing infections that lead to pus. In some cases, conditions other than bacterial infections can produce skin pimples or bumps that have the appearance of pustules. These include chickenpox, yeast infections, and herpesvirus infections such as cold sores or genital herpes. The formation of pustules may be a symptom of a bacterial infection, which may be a serious condition. Seek prompt medical care if you have pustules that are persistent or recurrent or that cause you concern.