What is pus? Pus is a thick, whitish to yellowish material composed primarily of dead cells that generally forms as a by-product of bacterial infections. The inflammatory cells that participate in the body’s immune response at the site of an infection eventually degrade and die, creating the substance known as pus. One of the most common types of bacteria that cause pus formation is Staphylococcus aureus, although any bacterial infection may produce pus. An infection that leads to the production of pus is called a purulent infection. When pus forms within enclosed spaces in the tissues, it causes abscesses. When it forms on the skin surface, it causes lumps known as pustules or pimples. Pus can also form when infections develop in internal organs, such as the bones, brain, lungs, and gastrointestinal tract. Because the formation of pus usually indicates a bacterial infection, people with conditions that weaken the immune system have a higher risk of infection and subsequent pus formation. The formation of pus generally signals a bacterial infection, which may be a serious condition. Seek immediate medical care (call 911) if pus formation is associated with other symptoms of severe bacterial infections such as high fever, headache, severe pain, difficulty breathing, or confusion or loss of consciousness for even a brief moment. Seek prompt medical care if your pus is persistent or causes you concern.