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 reddish or purplish, 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

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)
• Reddish or purplish, 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

Carbuncles

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
• Recurs

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.

Your doctor will likely be able to diagnose a boil or carbuncle simply by looking at it. A sample of the pus may be sent to the lab for testing. This may be useful if you have recurring infections or an infection that hasn't responded to standard treatment.

Many varieties of the bacteria that cause boils have become resistant to certain types of antibiotics. So lab testing can help determine what type of antibiotic would work best in your situation.

You can generally treat small boils at home by applying warm compresses to relieve pain and promote natural drainage.

For larger boils and carbuncles, treatment may include:

• Incision and drainage. Your doctor may drain a large boil or carbuncle by making an incision in it. Deep infections that can't be completely drained may be packed with sterile gauze to help soak up and remove additional pus.
• Antibiotics. Sometimes your doctor may prescribe antibiotics to help heal severe or recurrent infections.

For small boils, these measures may help the infection heal more quickly and prevent it from spreading:

• Warm compresses. Apply a warm washcloth or compress to the affected area several times a day, for about 10 minutes each time. This helps the boil rupture and drain more quickly.
• Never squeeze or lance a boil yourself. This can spread the infection.
• Prevent contamination. Wash your hands thoroughly after treating a boil. Also, launder clothing, towels or compresses that have touched the infected area, especially if you have recurrent infections.

You're likely to see your family doctor or primary care provider first, who may then refer you to a specialist in skin diseases (dermatologist) or infectious diseases.

What you can do

List all your signs and symptoms and when they first occurred. Record how long the bumps lasted and if any recurred. Make a list of all medications — including vitamins, herbs and over-the-counter drugs — that you're taking. Even better, take the original bottles and a list of the doses and directions.

For boils and carbuncles, some basic questions to ask your doctor include:

• Are tests needed to confirm the diagnosis?
• What is the best course of action?
• Is there a generic alternative to the medicine you're prescribing?
• Can I wait to see if the condition goes away on its own?
• What can I do to prevent the infection from spreading?
• What skin care routine do you recommend while the condition heals?

What to expect from your doctor

Your doctor is likely to ask you a number of questions, such as:

• What did the boil look like when it first started?
• Are your symptoms painful?
• Have you had a boil or carbuncle before?
• Are you having fever or chills?
• Do you have artificial heart valves, joints or other implanted devices?