A boil (furuncle) is an infection of a hair follicle. A carbuncle occurs when a group of hair follicles next to each other become infected. It is like a multiple boil. Chronic furunculosis is a condition where you have crops of boils that occur over a longer period of time.
Small boils are very common and often go away without any treatment. But they can be irritating and can also cause distress if they are in a prominent position such as on your face. Larger boils usually need treatment. If you get lots of boils that keep coming or don't go away then you may need some tests to check if there is any underlying cause.
Who has boils?
A boil looks like a small red lump on the skin that is tender. The surrounding skin may be swollen and inflamed. Thick, infected fluid called pus fills the centre of the boil. Boils can develop in any hairy area of skin. In particular, in sites where there is friction, or where the skin is sweaty. For example, they can appear on the neck, face, armpits, arms, and buttocks and around the back passage (anus). A boil in the ear canal may be very painful. Carbuncles most commonly develop on the back of the neck or on the thighs.
A boil or carbuncle can occur in anyone at any age. However, they are uncommon in children and most common in teenagers and young adults. You have a higher risk of developing a boil or carbuncle if you:
- Have other skin conditions that may cause you to scratch and damage the skin - for example, eczema or scabies.
- Are very overweight (obese).
- Have a poor defence against infections (immune system).
- Have an illness making you frail or generally unwell.
- Are a carrier of staphylococcal germs (bacteria).
What causes boils?
Boils are caused by bacteria (germs) multiplying below the skin surface. Your immune system is important in helping you to fight off infection. That means you have a higher risk of developing a boil or carbuncle if germs get under your skin and your immune system does not kill them quickly enough to stop them multiplying.
If you develop recurring boils (chronic furunculosis), your doctor may suggest some tests to look for an underlying cause. For example, to check if you have a poor immune system. You may be prone to boils just because you're generally run down and tired. But it's important to check there isn't any underlying problem, such as type 2 diabetes.
One cause of recurring boils if you are otherwise healthy is that you, or someone in your family or household, may be a carrier of staphylococcal germs (bacteria). This means that a certain number of these bacteria live harmlessly on your skin, or in your nose. If you are a carrier, you tend to be more prone to skin infections and boils. In particular, these bacteria may quickly invade and multiply in broken skin following a minor cut or injury.
Treatment with antibiotics and/or antibiotic nasal cream may clear staphylococcal bacteria from carriers and reduce the chance of boils, or other types of skin infection, from coming back.
What is the treatment for a boil or carbuncle?
- Small boils (furuncles) may subside and go without any treatment. You can ease pain by covering the boil with a flannel soaked in hot water. Do this for 30 minutes, 3-4 times a day. (Be very careful that the water is not too hot. It is important to avoid the risk of burns from hot water.)
- Larger boils and carbuncles are best treated by letting the infected fluid (pus) out. Sometimes this is done by a doctor who drains the pus, using a needle and syringe. Sometimes a small cut in the skin is needed to let out the pus (incision and drainage). The wound is covered with a dressing until the skin heals. The skin usually heals quickly once the pus has been drained.
- A course of an antibiotic medicine, such as flucloxacillin, is sometimes prescribed in addition to draining the pus to help clear the infection from the skin.
What will happen if I do nothing?
Typically, after several days (sometimes after a week or more) the boil will burst and pus will leak on to the skin. The pain tends to ease when the boil bursts. Once the pus has gone, the infection in the surrounding skin tends to fade away gradually over several days. A scar may be left at the site of the boil.
A carbuncle tends to increase in size for a few days - up to 3-10 centimetres across, sometimes more. After 5-7 days, various pus-filled lumps appear on the surface and pus leaks on to the skin. It then may break down and form an ulcer on the skin surface before the infection gradually subsides. The skin then gradually heals leaving a deep scar.
While infection and pus are in the skin, there is a risk that some germs (bacteria) may spread in the bloodstream to cause infections in other parts of the body. This is uncommon but, if it happens, it can lead to serious infections of a bone, the brain, or other parts of the body.
Further reading and references
; NICE CKS, January 2017 (UK access only)
; Management of common bacterial infections of the skin. Curr Opin Infect Dis. 2008 Apr21(2):122-8. doi: 10.1097/QCO.0b013e3282f44c63.
; Incidence and recurrence of boils and abscesses within the first year: a cohort study in UK primary care. Br J Gen Pract. 2015 Oct65(639):e668-76. doi: 10.3399/bjgp15X686929.
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