The most frequent cause of carbuncles is the bacteria, staphylococcus aureus. A carbuncle is a collection of skin boils (furuncles). A boil (or furuncle) is an infection of a hair follicle that contains a small collection of pus (referred to as an abscess) beneath the skin. Typically, the infected mass is filled with fluid, pus, and dead tissue. While a boil is single, small, and round, a carbuncle is a group of boils under the skin that is red, swollen, and painful. Besides the back and nape of the neck, other areas of the body such as the buttocks, thighs, groin, and armpits are also susceptible to developing carbuncles.