A boil is a red, swollen, painful bump under the skin that is caused by an infection. Boils often start in an infected hair follicle. Bacteria form an abscess or pocket of pus. Eventually, the pus may come to a head and drain out through the skin. Boils can occur anywhere, but common sites include the face, neck, armpits, buttocks, groin, and thighs.
There are several types of boils:
- Furuncle or carbuncle—an abscess caused by the bacteria Staphylococcus aureus or Streptococcus pyogenes, sometimes occurs as several boils in a group
- Pilonidal cyst—an abscess that occurs in the crease of the buttocks almost always require medical intervention
- Cystic acne—an abscess that occurs when oil ducts become clogged and infected, more common in the teenage years
- Hidradenitis suppurativa—an uncommon disorder where multiple abscesses occur in the armpit and groin area