What is a boil? A boil, sometimes also referred to as a furuncle, is an infection involving a hair follicle on the skin. Boils are typically caused by Staphylococcus aureus bacteria, although they may be caused by other bacteria or fungi present on the skin surface. In addition to affecting the hair follicle, infections that cause boils will also typically involve the surrounding skin. Boils form when bacteria from the skin surface infiltrate a hair follicle. This intrusion of bacteria triggers an immune response that includes drawing white blood cells into the infected follicle to fight the infection. If the infection in the follicle is not cleared quickly, pockets of bacteria, dead cells, and fluids can develop, resulting in the formation of a boil. Boils may form individually or be grouped together. When multiple boils fuse together to become one lump, the resulting lump is called a carbuncle. Boils may occur anywhere on the body, although they are most commonly located in the armpits or on the buttocks, face, neck or thighs. Although there is no specific cause for the formation of boils, poor hygiene or conditions that weaken the immune system can lead to increased susceptibility. Before boils can heal completely, they must open and drain. Boils typically resolve on their own within two weeks of onset. Seek prompt medical care if a boil lasts for longer than two weeks, is recurrent, is located in the middle of the face or on the spine, or is accompanied by other symptoms, including a fever, an excessive amount of fluid around the boil, or the presence of red streaks coming from the boil.