An abscess (collection of pus caused by a bacterial infection) that is larger than a boilit may be as small as a peanut or as large as a golf ball. The infection arises from hair follicles. The most common kind of bacteria causing an abscess is Staphylococcus aureus. A more dangerous, antibiotic-resistant form of Staphylococcus aureus, called MRSA (methicillin resistant Staphylococcus aureus), is becoming more common. Abscesses can occur anywhere on the body but are more commonly found on the back, neck or in skin areas that are shaved frequently. The infection can be contagious and is more common in those people with diabetes or other disorders that depress the immune system.