A boil is a painful, pus-filled bump that forms under your skin when bacteria infect and inflame one or more of your hair follicles. A carbuncle is a cluster of boils that form a connected area of infection under the skin.
Boils (furuncles) usually start as red, tender bumps. The bumps quickly fill with pus, growing larger and more painful until they rupture and drain. Areas most likely to be affected are the face, back of the neck, armpits, thighs and buttocks.
You can usually care for a single boil at home. But don't attempt to prick or squeeze it — that may spread the infection.
Boils can occur anywhere on your skin, but appear mainly on the face, back of the neck, armpits, thighs and buttocks — hair-bearing areas where you're most likely to sweat or experience friction. Signs and symptoms of a boil usually include:
- A painful, red bump that starts out small and can enlarge to more than 2 inches (5 centimeters)
- Red, swollen skin around the bump
- An increase in the size of the bump over a few days as it fills with pus
- Development of a yellow-white tip that eventually ruptures and allows the pus to drain out
A carbuncle is a cluster of boils that form a connected area of infection. Compared with single boils, carbuncles cause a deeper and more severe infection and are more likely to leave a scar. People who have a carbuncle often feel unwell in general and may experience a fever and chills.
When to see a doctor
You usually can care for a single, small boil yourself. But see your doctor if you have more than one boil at a time or if a boil:
- Occurs on your face or affects your vision
- Worsens rapidly or is extremely painful
- Causes a fever
- Gets bigger despite self-care
- Hasn't healed in two weeks
Most boils are caused by Staphylococcus aureus, a type of bacterium commonly found on the skin and inside the nose. A bump forms as pus collects under the skin. Boils sometimes develop at sites where the skin has been broken by a small injury or an insect bite, which gives the bacteria easy entry.
Although anyone — including otherwise healthy people — can develop boils or carbuncles, the following factors can increase your risk:
- Close contact with a person who has a staph infection. You're more likely to develop an infection if you live with someone who has a boil or carbuncle.
- Diabetes. This disease can make it more difficult for your body to fight infection, including bacterial infections of your skin.
- Other skin conditions. Because they damage your skin's protective barrier, skin problems, such as acne and eczema, make you more susceptible to boils and carbuncles.
- Compromised immunity. If your immune system is weakened for any reason, you're more susceptible to boils and carbuncles.
Rarely, bacteria from a boil or carbuncle can enter your bloodstream and travel to other parts of your body. The spreading infection, commonly known as blood poisoning (sepsis), can lead to infections deep within your body, such as your heart (endocarditis) and bone (osteomyelitis).
It's not always possible to prevent boils, especially if you have a weakened immune system. But the following measures may help you avoid staph infections:
- Wash your hands regularly with mild soap. Or use an alcohol-based hand rub often. Careful hand-washing is your best defense against germs.
- Keep wounds covered. Keep cuts and abrasions clean and covered with sterile, dry bandages until they heal.
- Avoid sharing personal items. Don't share towels, sheets, razors, clothing, athletic equipment and other personal items. Staph infections can spread via objects, as well as from person to person. If you have a cut or sore, wash your towels and linens using detergent and hot water with added bleach, and dry them in a hot dryer.