We include products we think are useful for our readers. If you buy through links on this page, we may earn a small commission. Here's our process. A boil or furuncle is a skin infection caused by the bacteria Staphylococcus aureus . It tends to thrive in warm, moist areas of skin, most commonly the nostrils, armpits, the crease between the buttocks, and the inner thighs. Boils can also spread under the skin and cluster. If this happens, the clusters form deeper abscesses called carbuncles, which may scar the skin. The size and color of a boil can vary, depending on how severe it is, or the amount of skin involved. However, most boils tend to have a similar appearance. Share on Pinterest A boil is a rounded swollen bump that may be filled with light-colored pus. Boils occur when the bacteria Staphyloccus auerus invades the walls of a damaged or exposed hair follicle and colonizes it, causing infection. Some 10 to 20 percent of the population are carriers of Staphyloccus auerus . ...