Boils are very common. They are most often caused by the bacteria Staphylococcus aureus. They can also be caused by other types of bacteria or fungi found on the skin's surface. Damage to the hair follicle allows the infection to grow deeper into the follicle and the tissues under it. Boils may occur in the hair follicles anywhere on the body. They are most common on the face, neck, armpit, buttocks, and thighs. You may have 1 or many boils. A boil may begin as tender, pinkish-red, and swollen, on a firm area of the skin.