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. With time, 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 that is caused by bacteria, sometimes they occur as several boils in a group
  • Pilonidal cyst—an abscess that occurs in the crease of the buttocks and almost always requires medical treatment
  • 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

Pilonidal Cyst
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Causes of boils may include:

  • Bacteria
  • Ingrown hair
  • Splinter or foreign object lodged in the skin
  • Blocked sweat gland or oil duct