Boils, Carbuncles and Furunculosis

Boils and carbuncles are pus-filled infected lumps on the skin. They usually occur as a 'one-off' in a healthy person. Treatment commonly involves draining the pus, and a course of antibiotics. If you have recurring boils you may be advised to have tests to check for an underlying cause.

What are boils and carbuncles?
A boil (furuncle) is an infection of a hair follicle. (Most of the skin is covered with tiny hairs which grow out of hair follicles.) It is usually caused by a bacterium (germ) called Staphylococcus aureus. A boil looks like a small red lump on the skin which is tender. The surrounding skin may be swollen and inflamed. Pus (thick, infected fluid) fills the centre of the boil.
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 period of time. The boils can develop continuously, or occur from time to time.
Who gets boils and where do they develop?
Boils can develop in any hairy area of the skin. In particular, in sites where there is friction, or where the skin is sweaty. For example, the neck, face, armpit, arms, wrists, buttocks, and around the anus. 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 commonly occur in teenagers and young adults. You have a higher risk of developing a boil or carbuncle if you have:
other skin conditions which may cause you to scratch and damage the skin. For example, eczema or scabies.
obesity.
a poor immune system.
an illness making you frail or generally unwell.
you are a 'carrier' of Staphylococcus bacteria (see below).
What is the treatment for a boil or carbuncle?
Small boils 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.
Larger boils and carbuncles are best treated by letting the 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 antibiotics is often prescribed in addition to draining the pus to help clear the infection from the skin.
What if they are not treated?
A boil may get larger, and more tense and painful as more pus accumulates. Typically, after several days (sometimes after a week or more) the boil will 'burst' and pus will leak onto the skin. The pain tends to ease when the boil bursts. Once the pus has gone, the infection in the surrounding skin tends to gradually fade away 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 onto 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.
Whilst there is infection and pus in the skin, there is a risk that some 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.
Recurring boils (chronic furunculosis)
If you develop recurring boils, your doctor may suggest some tests to look for an underlying cause. For example, to check if you have a poor immune system.
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 Staphylococcus 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 Staphylococcus bacteria from 'carriers' and reduce the chance of boils, or other types of skin infection, from recurring.