An abscess is usually caused by a bacterial infection. Often many different types of bacteria can be part of a single infection. In the United States and many other areas of the world the most common bacteria present is MRSA (methicillin-resistant "Staphylococcus aureus"). Rarely parasites can cause abscesses and this is more common in the developing world. Diagnosis of a skin abscess is usually made based on what it looks like and is proved by cutting it open. Ultrasound imaging may be useful in cases in which the diagnosis is not clear. In abscesses around the anus, computer tomography (CT) may be important to look for deeper infection.